My 89 244 does that every once in a while. usually right after i start it up or at idle. i usually either pull up on the gas pedal or tap it and it comes down. It used to do it everyday until i cleaned out the throttle body, now it only does it when its cold out. Check the throttle body. my next thing is the throttle cable, mine is kinda messed up so htats on my list to replace. also, Idle control vavle right below the throttle body. It has two hoses (inlet/outlet) and a wire connection on the other end. clean that out, however if you use Carb cleaner, make sure it is all dry before starting. It likes to kill Catalitic converters(burns too hot, kills ceramic mesh). Yours will look like this.
--
Blue '89 Volvo 244 DL 5-speed w/ 179k