Miss fire when up to temperature

[Aidan]

Hi all,
I have a California 280sl 1971. I hardly ever use it (last summer I did 10 miles!) but it starts up first time every time and runs absolutely perfect until it gets up to temperature. Then is starts to miss fire very slightly, this is worse under load and the hotter it gets. If I cruise at 40 mph no problems. Back fires (small pops) under deceleration. I live in the UK and can't get any "merc" specialists to improve the way my car runs. Can anyone help, I though it maybe the automatic choke sticking. Thanks Aidan