Electrical conundrum on 1990 Porsche 944

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After about a half hour driving, the radio starts to shut off and on; "drives me crazy". The heater and a/c also comes on and off intermittently. Just replaced with new battery, but want to catch the root of the problem before I have to get another one.
(3) Answers
I can't readily account for the radio problem , check connections at the radio itself after checking fuse(s) at the radio and at the fuse box.
The a/c , heater fan issue is probably the ballast resistor failing. It is located under the plastic cowling for the vent box , passenger side , under the hood.
It sounds like there is a power issue. If the radio and HVAC are on the same circuit and turn off intermittently together then I would look at the wiring between the fuse box and components to see if there was a bad connection.
Not sure about this one but it could possibly be the alternator. i had similar problem in my car where the alternator would send surges through the circuits in my car (not porsche, holden) this wouold cause the headlights to blow, gauges to fail, all warning lights to go on and the radio to stop working.