Quark Woes --- Need help please

[Patrick]

My first 2 would cut out a few minutes into a race on 14 cells. (they stop for a few seconds, then go again, usually slamming on the brakes when they stop)
The first one had a lot of smoke coming out of it after doing this.
The second one did it as well, after every run there was a slight burnt electronics smell coming from it which gradually got worse every time I used it. Ater a couple of weeks this controller also only had about 1/2 power on 12 cells, but full power on 14 cells..it would just cut out though.
While I was trying to get this one replaced S&K told me they had been thermalling and I burnt them out, because I kept driving (it's hard to stop driving in the middle of a race though, even if you think somethings not right). I hadn't really considered that much though, because they never got that hot (I was using the heatsink from rc-monster, 1 40mm fan, and sometimes a 3" computer fan). S&K just said the outside temp will be less than the internal temp, which I already knew and it just made me think more about the Quark's poor heat transfer from inside to case.
At first I thought it might be glitching or the UBEC, but it didn't go away when I upgraded from the stock TQ3 (origional e-maxx radio) to a spektrum and except for when the second one was acting funny, all 3 don't seem to do it on 12 cells, so I'm kind of thinking they were thermalling.
I haven't used the 3rd one (got it back from S&K about 1 1/2 weeks ago) on 14 cells in a race yet, because I'm afraid it'll do it again.

This is in an e-maxx with a 7xl.