Strange Idle

[mark]

I have a strange problem with the idle speed on my 4.3 Chimaera. Sometimes when the car is rolling the idle speed drops to about 500. When I stop the revs come back up to about 900. Start rolling again and the revs drop.

Some days all is fine and I get a constant 1000 idle speed. The engine seems to pop and bang on overrun the days of the bad idle.

Any ideas?

Cheers Mark