Could be plug leads (is it like the car 'hits a brick wall' ?) can he gently accelerate past those rev's? Had same problem once before at the same sort of rev's.

Or possibly a sensor gone down.

EsCos runs a 'lektron' valve and you're right, it's different to an amal valve - PJ's do them, as would probably Amber, Woodford etc..

HTH

Bill