S80: Engine RPM hang/not stable

[urban60]

I have two issues, for which I believe are from the same reason.

First is the RPM hang. If I change (any) gear over 2000 rpm the RPMs will hang at cca 1500, but what I've noticed is that if I press in the clutch it will stay (hang) a lot longer on 1500rpm and then drop do normal as if I leave the clutch disengaged (difference is cca 1-2 seconds).

Second is the RPM fluctuation in idle. This happens if the engine is warm and is fluctuating for about +/- 100 rpm with low frequency. One time when I've stopped in idle, the engine rev'd to 1500rpm by itself.

Engine: D5244T5
Gearbox: M66
Mileage: 300.000 km
DTC: only p0672, p0673 (glow plugs on 3rd and 4th cylinder)

Any thoughts?

[urban60]

Updates:
Glow plugs were changed and new OEM fuel filter installed, throttle body clean, new battery, but that didn't resolved the problems.

1. RPM still hangs when shifting gears.
2. RPM still fluctuates in idle.
3. Rough idle only when hot (this is new one).
4. When cold and I start the engine, it would start but the engine hangs @700RPM for a few seconds and after that it jumps over 1000 (like it should normaly). At that moment, whole centre console is off and comes on only when jumps over 1000RPM. This one is weird.

I had ECM-P245202 (Diesel Particulate Filter Differential Pressure Circuit - General signal failure), but that has gone away and CEM-B10EA14 (Positive Temperature Coefficient Heater. General Electrical Failures. Circuit Short To Ground or open). I guess faulty diff DPF sensor could cause such problems? But I don't understand how the ECM won't log it again.

Maybe a vacuum leak? Could someone briefly explain where to look for vacuum leaks on euro 4 D5? I'm familiar with that on the euro 3 engine, but not on euro 4.

Milage: 360.000km