2000 Jeep Cherokee - Check Gauge Light

[petejeep2000]

Anyone:
It just seems like one problem after another: It started last month, the check gauge light popped on, and I quickly checked the antifreeze, oil, transmission, power steering fluids - all seem ok. I blamed the heat we've been having. BUT, that darn light appears after about 10-15 min of local driving. When I come to a complete stop - the engine oil drops from 40 to zero, and when I speed back up, the oil pressure level indicator goes back up to 40. I took this into Jeep - they replaced/removed oil pressure sending unit. This problem is still happening. WTF!! Any ideas on what to tell my jeep service to check for or replace. Any/all ideas will be noted. Thanks. Pete