My car had a misfire, rough idling and unstable rmp plus stalling. i took it to the workshop and they sealed the EGR which helped. The misfire reduced significantly and idling smoothed out however the rmp instability is still there…sometimes drops too low to about 250 with vibration this is however intermittent. to raise the rmp i need to step on the accelerator and it improves. Stalling has happened only once since. What could be the problem?