1996 Volvo 850 2.5 Diesel Lambda Sensor Light on and Poor Running

My 96 Volvo 850 Diesel (2.5 5 cylinder - same a Audi A6) with circa 200,000 Miles on the clock has an intermittent warning Light on the dash. It icon is triangular, yellow on colour with a green symbol in the middle. The light will come on for 30 seconds to a minute and then clear, sometimes the engine will also run smoothly, normally in slow traffic or at idle, again clearing after a short while, always when the engine is at normal running temperature.

As far as I can work out this is the lambda sensor (or O2 sensor) warning light. I have thoroughly looked over the exhaust system to try to find the O2 sensors and or the CAT, neither of which are there, not surprising as this is a 96 Diesel I thought. I suspect that poor fuel mix control is what my car is trying to tell me. I have had the injector?s cleaned and spry pattern check all OK. Also I have run an injector cleaning agent though the fuel system. Neither of these ideas cleared the problem. There is some black smoke from the exhaust on hard acceleration, but more than is usual for a diesel. Car has stalled whilst driving at low speeds a few times now. After restarting fault seems to clear for a while at least and then gradually returns over after few weeks, sometimes longer. Last week it stopped while my wife was driving, didn?t get a good response about it next time will result in divorce so need to get it fixed. Any ideas welcome.