The car starts fine- but getting it to move, especially from a full stop is very difficult. Struggles to accellerate, can only get it up to about 55mph- When driving on completely flat road, it feels like I'm driving uphill with the emergency break on. When I press on the gas, the engine doesn't race...

What could the problem be?

Your convert bad

Haha, this car has long been sold, but thanks! It actually had ended up needing a new fuel pump, though.