Try backing out your Governor Damper/Idle Pin, re-setting to the proper idle speed and then adjusting the Governor Damper/Idle Pin.

If the idle pin is run in to far it can casue similar syptoms to what you are having because it can push you into the zone in the Fuel Injection Pump where the Governor does not control the speed.

Look at the Rubber parts of the throttle linkages and depending on what type you have on the Fire Wall behind where the linkage pivots is a sort of a plastic ball called a bushing. That ball wears and in fact crubles away when it gets old.

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84 300D, 82 Volvo 244Gl Diesel