Low power and cutting off under load, what's up?

[Outlawcart]

Hello, been a while since I've been on since usually these carts are pretty stable...however a friend has just brought me his cart as it's been acting up some.

Later model Club Car DS 48V (6 8V batteries)

The issue is, under load the power cuts out. He has tested the batteries and showed about 52V, however 2 days ago he purchased all new Trojans which did not correct the issue. Still shows good voltage and a load test shows good here.

We had some suspicion of the speed sensor. I have test driven the cart and the speed sensor appears to be working to some extend because it beeps when drifting and no power, however there is no self braking applied. The motor magnets are not slowing the cart at all on a downward slope.

Also, compared to my cart it appears to be very weak even on level ground.

Instead of throwing parts at it, what are your thoughts with parts to test, check, or replace?