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1979 Volvo 264 Uneven RPM at all speeds

I have a 1979 Volvo 264 V6 with 87K on it. It runs uneven after it warms up (it has no problems when cold). After it warms up, the idle speed drops down to around 700 and then it starts to fluctuate. What it will do is stay down in RPM for a few seconds, and then suddenly go up a few hundred RPM and drift back down, and etc. This happens when idling or revving it. Sometimes it also hesitates a little when revving it. The strange thing is that the car only has this problem when the O2 sensor is connected. With it disconnected the car has no trouble running, hot or cold. If I disconnect the sensor while it is running poorly, it instantly returns to normal. This obviously points to the O2 sensor as being bad, but I’ve replaced it. The new sensor makes the car behave differently then before, but not good. With the old one the car would end up dying at idle, with the new one it just runs uneven.

I’ve done all the basic maintenance: new O2 sensor, air filter, fuel filter, oil and oil filter, and spark plugs. I’ve also checked for vacuum leaks and have not been able to find any, although I don’t rule it out. Any help is highly appreciated.

I’ve noticed that there is a buzzing noise that happens right before the RPM goes up. I believe it’s the frequency valve. The car will be running at a low RPM, and then it makes a buzzing noise, RPM goes up, and then the buzzing stops and the RPM goes down. Should the frequency valve always be buzzing? Some of what I’ve read online suggests it should. I’ve also read that the ECU is more likely to be the culprit then a bad frequency valve, so I’m starting to suspect the ECU has gone bad.