1996 Ford Thunderbird oil pressiure

Engine problem
1996 Ford Thunderbird V6 Two Wheel Drive Automatic 120000 miles

my oil pressure drops when I come to a light and then the check engine light comes on. When I excell and the rpms go up the pressure goes up and the light goes out, they replaced my oil sending unit or switch what ever you call it and it worked ok for two weeks. I went out of town the other day and it started doing it again. I have a mechanic telling me its going to cost me$750.00 to fix it, he said it sounds like my oil pump is bad. What do you think.