1990 corolla oil pressure warning light

I've owned this 1990 toyota corolla SR5 for about a year. Recently, the oil pressure warning light starting coming on, but only after driving continously for more than an hour and only while the brake pedal is pressed. I thought maybe it was some short in the wiring, but in conjunction with the oil warning light, it also idles differently, sort of pulsates or revs up on it's on and is a little louder. Drives fine otherwise. I check and change the oil regularly. It has about 135,000 and is due for another timing belt change. Could it be a timing issue? or something worse?

 

You have a rubber hose with a crack or it is broken outright. Most likly a vACCUMN HOSE.
When you slow down or sit at an idle it sucks on a vacuum and the engine slows down. If it slows down eneff, the oil pressure drops and turns on the light. When you rev it up or drive it, it runes fast eneff to keep the oil pressure up.

 

That sounds like a logical solution. But I checked out all the lines, couldn't find any leaks or breaks.

 

Not likely a timing issue. I suspect you have two issues; oil pressure warning and idling. For the relatively low cost I think I would replace the oil pressure sender on the off chance it's failing.

Idling I would try scrubbing the intake throat with carb cleaner and a toothbrush all around the throttle plate and both sides of the plate itself. See if that helps.