Having a weird error: Using a stimpack from an active item slot costs 5 action points instead of 2--and if I don't have enough, it still drains the remaining action points from my bar. Anyone else encounter this? I'm running Vista, btw.

I have run into this problem a few times as well. I tend to die sometimes and when I reload it works normally.

First of all, you should NEVER equip stimpacks - you can use them from the inventory itself. It costs 4AP, but you can use as many as you want for those 4 points. If you need to heal 100 HP, you can do that in one turn. Just right click on the item and drag down to the use icon.
You can also use this time to reload your weapon if you had fired off a few rounds.