Cooling system pressure holding at 30 psi is a good sign, despite the seeming contradiction presented by the bubbling in the coolant reservoir. Is the coolant heating up very quickly after a cold start, or is the engine getting hotter than normal? Those would be symptoms that go hand and hand with exhaust pressurization of the cooling system. I'd get a test kit to verify the presence of exhaust in the coolant before pulling the head if this were my problem. A compression test won't hurt, but it also won't give a clear diagnosis, regardless of the results.

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'79 240D