1990 Jetta Carat engine's rpms fluctuate between low and high rpms

[adammarker]

My VW Jetta Carat has over 190,000 miles and is experiencing a problem that will drop the rpms down to zero and back up to higher pms, while driving and standing still. The engine will eventually stall out while standing still.
There is no apparent smell of gas, burning or black smoke from the exhaust.
The car is at the shop and they have replaced the valve cover to breather grommet but it did not stop the fluctuating rpms. Then they replaced the cap and rotor, but this did not resolve the problem. They are still searching for a solution.
The week before, the car was driven on a major highway for about 2 hours at a constant speed of 70mph and worked very well. The outside temperature was about 90 degrees. Ironically, as I was pulling into the inspection station several days afterward, the engine began to stall out consistently and they failed the car. It was at the station to retest for not meeting the high speed emission test, which was fixed by a VW dealer several weeks back.

[mechanic1980]

try replacing the dipstick and tube for the oil