oil pressure light 92' ES300

Whenever I drive my car (92' es 300, 112k miles) on the freeway and I hit 80+ mph, the oil pressure light would flicker on and off, and it would be very random, sometimes it would flicker on for a second then turn back off, then 5-10 minutes later it would flicker on but for say, 1 minute, then turn back off. This only happens when I drive around 80 mph, whenever I am going 60-70 it doesn't do anything. I've been told it could be anything from a faulty oil pressure sender to a broken oil pump, but I'm trying to figure out specifically what it is. It has started for about a month now and hasn't really bothered me since I rarely drive that fast anyways but I just started a new job that is pretty far and the freeway to get there flows at 70-90 mph. Can anyone help me out? :confused: