My girlfriends dads rev 2 tubby keeps stalling.

It starts OK and will run fine, but when it is warm and pulling up to the light for example, the revs go down to about 500 rpm then it stalls.

Apart from that it runs lovely.

This only started to happen after the battery was replaced.

Didn't know if it might have something to do with resetting the ECU or whether the battery is not good enough for the car.

We have checked the AFM and that is not clogged.

Does anyone know what rating the battery should be for a tubby?