poor acceleration when hot

I have a '79 911 SC that stumbles under full throttle when hot. It is fine for the first 30 minutes or so but as the engine gets hot it gets worse. If you lift off the throttle a litte, say to 80%, power comes back albeit less than maximum. The fuel pump and warm up regulator have both been replaced and both helped but have not solved the problem. The engine has had a tune up recently and runs fine otherwise. No idle problems when hot and no over heating or anything like that. Thanks for help anyone, I have been working on this (read spending money) for months and can't figure it out.