1992 240 150,000 miles - Purchased from original owner at 120,000 miles.
Occasionally ( once every few months month ) the engine will not idle. It appears to be dragging a cylinder. I put my foot on the brake, keep it off idle and get it moving. Then the problem goes away and it runs sweet for two or three weeks. Then, at a traffic light, the problem returns. Two minutes later, it's running sweet again, purring like a kitten.

Plugs, wires, rotor and distributor are 3 years old.
Throttle was cleaned at 80,000 miles by dealership.
Throttle switch is apparently original.
I put in a bottle of NAPA fuel injector cleaner(first time) at my last fillup. This tank is now used up and I still have the problem.

Suggestions??