I've tried that now and the mission failures still appear.

Compatibility mode should force it to run on one core, so I'm not sure what else could be causing the mission triggers to break. Maybe speedstep?

The game should use just one core anyway, since it was released in 1998 and at this time there was not even hyper-threading, let-alone multi-core cpus (ok server-boards with multiple sockets, but that's another thing). Usage of multiple cores has to be programmed deliberately.
I can have a look at the BIOS settings and deactivate one thing or another there for testing though, I'll pst the results here.

Damn i was hoping that H2IWclassic compatibility solution would work. It did not work. Thanks H2IWclassic for your help and trying to find solutions.

Multi processor systems did exist back then, but I was referring to the effect that can be seen in games such as Unreal Tournament where the program bounces between cores and speeds up abruptly due to changes in processor clock.

I've been getting this problem too, although I'm using an i5-3470 (4-core). It appears for me about 2-3 hours in, which is lucky because I can usually complete missions before this. Not sure if game speed has an effect; I'm using about medium speed IIRC.

- Bringing up pause menu still counts towards time
- Completing one mission and moving onto another still counts towards the time; to be safe restart the game before attempting any long missions

For those having problems completing missions quicker, here are some tips:

- Attack the tankers! Compared to CnC the economy is ridiculously expensive to set up; killing tankers will severely cripple enemies.
- Set up two oil/stations and put two tankers on each for the ideal economy
- Towers are way OP in this game! Find out where the AI is attacking, either seal off with walls or put a tower or two behind walls. That's all the turtling you need.

I been getting this too,

got one on the mission Convey after building a mobile outpost didn't like that all.
got a lot of them on a mission in the west fighting series 9's the surfing mission?

a link or a bug update for these issues would be quite welcome

I have this problem as well. To my experience, this ONLY happens to me whenever I bring up the in game options. (Esc button) I'm not 100% sure if It happens when the menu is simply opened, but I KNOW it happens whenever something within that window is used, Such as save, load (if I load a save game it's almost always a guaranteed failure sine you obviously have to open the menu to save it and it remembers) mission briefing, etc. More or less as long as I don't hit Esc, I don't get the random mission fail. I hope this provides some new insight to anyone working to resolve this. Also, I've noticed whenever you hit Esc the cursor gets really choppy, like the fps drops exponentially. Again, not sure why this happens. If anyone comes across a solution, please inform us?

**Edit**

Okay I tested what happens when you hit the ESC button if nothing is used within the menu, after a certain point (roughly about when you would get the mission failed after actually using an option in the menu) it won't let you build any units. At least for me. And this is not due to a maximum unit amount in any way (the bar is still green). Hope this helps at least a little.

Having the same problem with random fails. I'm guessing there's no solution at this point then? I always thought there was a patch that solved it, but going to Melbourne House page it doesn't seem available anymore.

It was really annoying but I managed to get by it : see post #10. Save frequently and when it happens, exit the game completely, run it again and load your latest save before you got the Mission Failed. That worked for me and the one time it didn't work, I followed the same procedure except after I loaded my latest save, I saved again and restarted the game for a 2nd time to load that save.

Luckily I only encountered the bug on about 2/3 levels.

Unfortunately not working for me; once a save has ended with mission failed, it will always end with mission failed, though not at exactly the same time.
I can sometimes get around it by exiting and starting the mission over, but not always. The last series 9 mission is proving particularly troublesome, from 5 attempts so far all have failed, plus random CTDs and "failed to create unit" errors.

After contacting support they basically said reinstall and play about with compatibility settings(I've tried all), but there's definitely a huge bug in the code somewhere. :(

that sucks man :( So restarting and loading a save from just before the mission failed doesn't work? Have you tried to restart -> load save from just before 'mission failed' -> save game again in new slot, say for example 'level nine' -> restart -> load that save, i.e load 'level nine'?

Yeh, it basically seems like it decides at the beginning if it will trigger or not and I just have to play and hope. I've tried loading different saves, saving again, no matter what it just suddenly pops up. But then I can restart the whole game, play the mission again from scratch and it's ok.

Unfortunately GOG can't replicate it, even with a save. No idea what it is though because it's an in game message, it doesn't make sense that anything outside the game would cause it.