Although the HC was within limits, it looks high at low speed compared to my '93.

Thus it looks to me like a slight misfire at low speeds, quite possibly caused by a slightly lean running condition.

Since it improves at higher speeds I would first look for vacume leaks. There are plenty of hoses and such on the vehicle which can deterioriate with age.

You really need to be able to hook up to an exhaust gas tester while checking things and try to find what makes it get better.

Standard tune up procedures are not likely to fix the problem. With any luck perhaps the O2 sensor and cat are ok. Your readings are not off the chart, so I would not assume a bad cat just yet.

You are probably going to have to turn the car over to an experienced and talented tech who is familiar with this situation on 240s and who has the right equipment. Either an independent specialist or a good dealer is called for. By all means do not go to someone who doesn't have lots of experience with red block Volvos and emission controls.

John