Intermittent RPM skips/drops (VIDEO)
 

Hi,

I own a Volvo S40 T4 phase 1, 1999. It has always been in my family since new and I'm the third owner, has only run ~120K miles. Cared for, regular maintenance.

About a year and a half ago I first noticed some RPM skips while driving. Stopped at the side of the road and allowed the RPM to drop to idle I see what is seen in the attached video. It went on like this for 30s-1min and then it was fine to then appear and dissapear again a few months later, another few weeks and then another few months.

Battery, spark plugs and ignition coils are new.

Any idea of what could be causing this?