Check and/or replace all the rubber ends on the vacuum lines in the engine compartment. They either get soft and sticky or rock hard, and leak in either condition. This is the cheap fix if you find a bad one.

Also, check the idle valve hoses if you have CIS injection (I don't know what year it changed to LH) -- if these hoses are bad, you will have a serious vacuum leak.

This will also fix idle speed variations and rough idle problems.

If all the rubber bits are in good shape, connect a hand vacuum pump to the transmission modulator line and see if it holds vacuum. If it doesn't, you need to replace the modulator.

Very late shifts and light throttle downshifts are an indication that modulator isn't getting a good vacuum signal from the manifold.

Also check the electric kickdown switch under the accelerator pedal -- if it is stuck or shorted, the tranny is getting the kickdown signal all the time, and will downshift at speed.

It is also possible that the solenoid spring is broken, but that won't be intermittant.

Mostly likely you have some bad rubber hose ends -- when they get hard, they can seal sometimes and not other times. Don't forget to check the rubber hose end on the modulator, too!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!