IwubCheeze: I just finished Gothic 3 and found I had the same problem but when I saw this happening, my stamina bar was empty. Maybe this is the problem?
I can confirm this is not the problem. It generally happens as combat opens, then once I get going the RMB malfunctions less and less.
IwubCheeze: When I was using one handed weapons, I never had to worry about stamina except when running long distances. But when I switched to two handed weapons, my stamina bar was pretty much always empty even after a few seconds in combat. I used 20 LPs to boost it to 600 too and I was still running out rather quickly (especially in mobs) but I did notice I have a little more staying power if the enemies had a bit of distance from each other. It seems to me that stamina and two handed weapons go hand in hand. If you have an empty stamina bar, two handed weapons lose a fair chunk of their reliability.
On the subject of 1h vs 2h weapons, I still preferred 2h because the range of 2h weapons helped ensure I wouldn't get hit in 1v1 or 1vs small groups. Larger groups were always a problem in melee so I chose too run away and use my bow instead. Don't know if the problem is me but I found using 1h weapons risky until I could use a shield but when I got to Varant, I was always getting my ass kicked when trying to use 2 1h weapons or sword and board combo, 2h weapons helped a lot, assuming my stamina bar wasn't empty.
Not sure how much you monitor your stamina bar in combat (not that I have any judgments on anyone's playstyle!) but the spin attack (Hold RMB + Tap LMB) drains 50 stamina every time you use it, even if it bugs out or gets jammed. This is why stamina is dramatically more important to Two-Handers.
TBH I've been switching my stance a little bit too. I am also finding I get cheap-shotted by guys near my target when I use two-handers much less.
On the other hand zombies are a nightmare to face with Two-Handers...
Interestingly I'm going to say both 1h and 2h styles have their place. I'm a little impressed (not with RMB malfunctioning but everything else).
EDIT: Also I'm LOVING playing a mage. Once you get Mana Regeneration and a few hundred mana it's so great. I often blast enemies with magic, then switch to my staff for a few spin attacks, then when my stamina is lower I back off and switch back to magic.