1987 745tic w/ 163k miles new to me. I bought this car as is with a great body and running well but with a seized power steering pump, leaking water pump and a blown top radiator hose. I reasoned that when the power steering seized the water pump stopped turning( there was no belt on the power steering/ water pump/ crankshaft) and the rad hose blew (it was old). I have replaced power steering pump, water pump and some hoses(remainder to follow). The car runs very well with no sign of water in oil or oil in water and has tons of power and a very steady idle(no skips or missing). Now the problem, with the grey rad cap on the car will start and not leak any coolant visibly but after a 7 mile test drive I was down coolant. This morning I tried a black cap I have for my 242t project and upon startup coolant sprayed out from under the cap (I was watching as wife started car). But as soon as the car was running coolant stopped spraying and returned to normal running. There is no sign of bubbles percolating up in the reservoir as the car is idling. Does this sound like a head gasket to anyone or any other ideas? Thanks in advance.

Dan