I replaced the fuel filter which was clogged pretty bad and flushed the injectors which were in the same condition.The distributor cup was worn out and needed replacement along with the HT leads.One of them did 'nt work properly which resulted in one of the cylinders not working properly as well.Result: poor engine idle power.Now the rev. needle just points to the correct idle speed without even thinking of bouncing.The engine is more alive ,working with four cylinders:) and i also replaced the lamda sensor which was perfoming bad.I am happy that there was nothing like bad ecu or idle speed regulating valve and dissapointed the way my father had serviced the car.Thanks arno and long live VOLVO.