Before checking the throttle switches themselves:
Check the throttle linkage to make sure everything is actually returning to full 'idle state' even when you release the accelerator very gently. Sticky linkage bits seems to be a very common problem.
Can be intermittent, because it may only stick when the throttle is released gently, thus having less spring momentum to overcome the binding.
By linkage, I mean all the related parts, including the little balljoints, plastic wheels, and microswitches. The little plastic wheels seem to be particularly prone to binding.
I use a solvent, such as spray carb or brake cleaner to clean, and spray lithium grease to lube.
Board consensus says to use ATF to lube linkage, but I'm a non-conformist.
Best of luck.
1986 300E 5-Speed 240k mi.