Thanks for the reply, I removed the intake manifoil a while ago to switch the glow plugs, reinstalled it with new seals but of course it may be a gasket that did not fit properly, but I had the problem with rpm before I changed glow plugs but perhaps to a lesser extent.
Suspect that the air mass meter may have something to do with the problem because the problem has varied slightly depending on whether I have used the new or old air mass meter.
I will try to read the pressure on the manifole and fuel pressure today