low oil pressure warning, oil gauge registers around 80psi

I just recently purchased this 2004 Chevy Avalanche with 65,950 miles on it. The third week of driving I noticed the oil pressure gauge up around 80 psi, when I had to apply the brakes a bit hard the oil pressure would dip down to approx. 50 psi and then the low oil pressure warning would come on. I have the checked the oil level and it is perfect, there is no noise or knocking in the engine at any time, the truck runs perfect. I have check other venues for this problem and every thing or every one points to the oil pressure sending unit or the oil pressure switch.

I'm looking for some answers and or suggestions, I am mechanically inclined and have work on my vehicles in the past, also I do know what my limits are.

Eratic oil pressure from 0 to 120 lbs occasionally registered properly. Encountered a stall after driving approx 50 miles with 0 pressure showing after waiting approx 5 mins engine started and oil press gage acted normally.

The mechanic I had been working with was not available took to another mechanic and he charged $480.00 to replace oil pressure sender and manifold pressure sensor this fix lasted one day. Took vehicle to a Dealer he kept vehicle 5 working days and said he could not find any problem or get vehicle to act up when I brought vehicle in the check auto light was on also. Dealer charged $220.00 and said he could not find anything wrong, he did state the oil pressure gage acted up the first day but he could not get it to do it again. All 3 parties working on vehicle stated a Code 50 showed up but no one could resolve it.