DRIFT - this is sounding very much like the same problem I am having with my buggy that I just posted about.
http://www.rc-monster.com/forum/showthread.php?t=13040
What tires are you running? I noticed my problem seemed to be worse when running the Moab 1/8 buggy tires compared with others - there seems to be a 0.5" ballooning effect going on with these tires which will throw the overall gearing off when at WOT.
Also - where and how are you running? Are you racing on a track or bashing around in parking lots/dirt/grass fields?