'85 280ce losers power and then stalls

[princefari]

i'm trying to isolate the problem that causes the following: the engine on this car turns over beautifully and she always starts first time. however, when the engine is warmed up after a couple of miles she starts to lose power when the driver tries to accelerate (at c50mph) and eventually slows down so much that she stalls, sometimes backfiring for good measure. when the engine has cooled she starts again fine but the same happens. there is no problem when reversing, even when she has stalled going forward. can anyone throw some light on this?
thanks in advance