I'm having the exact same problem in my 90 2.5..Can't figure out what's going on. Except, If i am easy on the brakes for about 15 or 20 second, say rolling to a stop sign, my brake light will come on, or when im sitting at a red light for anything more then 20 seconds. I was also thinking brake booster, what manual tells you how to test it? The original Jeep manual, or a repair manual? I don't have the original one. I'm gonna check the booster tomorrow. If i come across anything else i'll let you know!