3.9 v8 k reg classic starting problem

My classic 3.9 v8 starts perfect from cold, but if you drive a couple of miles and the engine warms up, say you stop at the petrol station then re start the car, the engine fires up, the revs go up to about 1500rpm drop right down to practically nothing, then it will either cut out or will recover its self to normal tick over. Any ideas??? Happy new Year to you all.