I wouldn't put my good road bike on a trainer--it gives me the willies. Instead, I've got an old bike on it, and maybe this would have happened anyway, but now the shifters are shot. The good thing about using an old bike is that it matters less when things go wrong, and you can go for the cheapest fix available. Also, the weight of the bike is not a factor. Just keep looking around for an old bike--garage sales, bike shops. Even if the bike isn't shifting, you can always put a shifter on the downtube for about $6 (since you won't be needing the sti, really).