As mentioned before I use the Sabertooth 25A in Pilgrim the weight is at least 60kg never had a problem, but it is slower than a Dalek.

That's a sentence I thought I would never write!!

I would perhaps put forward the use of a Victor 833 I think the range is about 60A and is fan cooled! I use one in the lifter in Cherub and seems fine.

A few questions
what is the specification of the motor, drive ratio ,wheel diameter calculated speed? do you use the acceleration ramp on the speed controller? is it over volted?