I recently ran into a 78 slc with similar symptoms. The catalytic converter was toast and needed to be repalced. Excessive exhaust backpressure was the tipoff on the converter. After fixing the one problem the car still ran without power. Replacing the fuel injectors resolved the problem. The opening pressure of the CIS injectors had gotten to a point where they restricted the air into the cars engine. The air flow measuring point in the CIS injection was holding the air back because of the injectors would not open! If you are not familiar with the fuel injection system please let a shop look it over. It could also be a simple problem with the warm up regulator not doing its job. If the system control pressure is too high this can also cause the symptoms you are talking about. John