Cart goes into limp mode after one ride, Need help!

[bobotron]

I posted in the Star Cart board, but got no response. But maybe you mechanically inclined users can point me in the right direction anyways.?

I've had this problem with our cart since we got it about 6 months ago. It's repeatable so I feel like someone has a good guess about what is happening...

So I charge the cart fully overnight, drive it about 5 miles or so (to town and back) and about the time I get home or close to it, it goes into limp mode and drives very slow. I get a code 1,3 (speed sensor, but we've replaced that twice). Sometimes I can reset it with the run/tow switch, and it works for a little longer. Sometimes not. I can usually tell its about to do it because as I accelerate from a stop, it will slightly hesitate a couple times as it gets up to speed. For just a split second. And if it hasn't done it by the time I get back home, I can usually stomp on the pedal, and it will pretty much immediately happen again. (Doesn't happen when I stomp the pedal down with a full charge).

I'm not even sure how much of this cart is factory and what has been modified, so here is what I know:

2007 "Cruise Car SX2" (Star cart?) (I think it started as 36v)
48v system with 6 new 8v batteries (All test to about 9.1 volts even after this happens).
New heavy duty solenoid
Supposedly rebuilt 36v motor DE4-4011 from Advanced Motors in NY
Has a 48v Yamaha charger and port
Curtis 1268-5403 controller which I sent off to be tested and they said it was good.

Any ideas? Maybe the throttle signal? Or something with the motor? It has to be something directly related to the amount of battery power available after one drive. Small drop in amperage is triggering something?