Volvo 440, idling problem

Hi, my (1989) Volvo 440 GLE injection 1721cc engine has gradually developed an idling problem whereby the engine revs drop & the engine sometimes cuts out or occasionally will rev higher (1200+) for a short length of time. This problem happens more often when the engine is warm & when braking coming up to traffic lights or a junction, rather than when the car is in slow moving queues of traffic. However on some days this problem will only happen occasionally whereas on other days will happen continuously. I have changed the plugs, HT leads, air filter & added some injector cleaner, but non of these seems to have had any effect. can anyone here suggest what I should look at next to cure the fault. Thanks in advance, Stephen