of course since this problem was posted in May 08, and it's now October 08, I hope you haven't left this unit in the corner and burried it under other trash.
The first thing _I_ would check is the repair company's phone number is in the waste bin, and then check the yellow pages to find someone that has a clue to repair your (not new) unit.
This is not easy to guess about this problem, it could be related to the chopper controller or the accel pedal and it's switches. it does sound as if it is getting what we call a "pulse monitor trip", where the rectifiers are not correctly pulsing, one of the 3 major SCRs (silicon controlled rectifiers) or the related large diodes is shorted closed. but on this old of a unit, this could even be caused by a low battery charge.