PIP sensor is what the EEC uses to measure the RPM of the engine. Like the guy above said, it sounds like your TFI module is going bad and needs to be replaced. Its located on the side of the distrubutor and has a wiring harness going to it.
Don't be so sure. I just fixed this same problem in another recent thread. TFI module was fine, the PIP (Hall sensor or pickup coil) inside the distributor would go bad after getting to engine operating temperature, basically getting heat-soaked.