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I just fixed the same problem on the 245. It wasn't intermittent, just zero timing adjustment under all conditions. It should advance with the vacuum signal when the throttle first opens, and with rpm's. Most cars I've owned ended up with a total advance in the mid-thirties around 3000 rpm's, and when your timing is stuck at 10 degrees, it certainly makes sense that around 25 degrees difference in ignition timing is going to make a difference. Replacing the computer fixed that instantly.
Anyways, it's unlikely that more than one part of the circuitry adjusts the timing, and when that goes, all adjustment goes with it. I'm betting that the rebuilt computers simply have the usual trouble spots fixed, and they're out the door if they pass a test. But these computers are getting old, and resistor values can drift over time, electrolytic capacitors dry out, ceramic capacitors oxidize or short, so forth. The components might still work at the shop, but they're 20 years on the way to kaput. IIRC, the delay/advance circuit should rely on at least one capacitor, probably ceramic. And that's as far as my electronics knowledge goes! It's been a really long time since electronics class. Maybe someone more skilled can track it down.
Are new computers available? If so, and assuming they're manufactured recently, that should be a good long term solution.
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