DVI / VGA Game runs faster on DVI

hi, i have 2 games ok, sonic heroes and sonic riders, now the thing is sonic heroes game lags when my monitor is running through VGA but its perfectly fine on DVI, but for sonic riders, its the other way around... why? any reason? because i don't really wana have to keep swapping cables
thx

btw, can i just ask, lots of peoples sig's have those gif's in them that change and say stuff like "demonoid user" "futurama fan" or stuff like that. where do you get them??