yeah, i have experienced the exact same thing. everytime i do sprint intervals i end up pulling the rear up. it gets kinda scary sometimes as the bike gets squirrely. talked to some people about it and they said it might be attributed to pulling up real hard on the upstroke. maybe that and the addition of pulling hard on the drops unweights the rear. i avoid it by doing sprint intervals up hill, but i gotta get back to it and fix it as it seems that there is a power loss because the rear loses contact with the pavement