Quote Originally Posted by DebW View Post
Does your cassette freewheel easily? If not, the bike would perform perfectly while pedaling but slow when you coast. Try lifting the rear wheel and spinning the wheel forward. Do the pedals turn when you do this?
That seems like the most likely culprit to me so far, although I still think anti-gravity is an option.

I have that same bike and do not experience the problem, but there could be a problem with how it was built.