on my p38 this happens when the car is sucking in air, usually due to the gas mixer ring working loose as it is held on by one grub screw(crap way i reckon) so have a look and see if there are any splits or loose fittings, the air intake pipes may also be collapsing though i would expect this to happen at higher revs.
you could try taking out the air filter and seeing if its resticting.
after that you got to look at whatever system is used to control idle speed.
bit vague but better than nowt i suppose!!!
