Have you taken out the air filter and looked at the accelerator linkage? It is all spread out to look at and it is pretty complex. There are many pivots and ball connectors. If you try moving the various parts, you should be able to tell if the problem is in the cable (from pedal to lnkage) or in one of the linkage pivots/connectors.

When I do an oil change I always lubricate the linkage since I have the air cleaner off to reach the oil filter. I use a Valvoline synthetic oil in a spray can with tube to get the oil right into the pivots and ball connectors.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog