This is quite a common thing in these engines and can be caused by lots of things not least cold damp weather and short runs. One thing that's fairly common and can aggravate the problem is the engine breather hose being either blocked with debris or collapsing under vacuum when the engine is running at mid to high revs. Old hoses can become soft and lose their structure when under vacuum so nothing can get through. I'm not exactly sure where it is on your engine but it usually runs between the top of the engine (Oil filler cap is common) and the air intake housing or filter housing. You can usually flush the gunk out with a good dose of engine flush and an oil change. If the tins of flush are small buy 2 or dilute them 50/50 with some diesel or kerosene and follow the instructions.
