it sounds like a problem for a professional to look at. ive experienced that with convertables it can and always will be something different. if could be anythiing from the motor, it not being in place properly.. unfortunately over the internet i cant help you so much as to guess at what it could be. my honest opinion is to save yourself time and grief and take it to a pro. good luck, and lemme know what you find.
by the way . welcome to the Forums.