Oil pressure issue

I have a 2008 Chevy tahoe LTZ that the oil pressure
at start will be about 20 then will go to 40 at about
2000 rpm but then abruptly drop to 0 and lights go
off for a brief second then pressure goes back to
40. This just started when weather started getting
below freezing. Checked oil level it is good and I
still have about 1500 miles before it is due for oil
change. Is this a oil sending unit issue or what?
Well sensor was changed screen cleaned under
sensor. Still setting off code. Dealership saying I
need new engine. Engine has plenty of oil there are
no new noises......belt makes a clicking noise as it
turns, but no other noises that are not normal. I
took it to a dealership where my wife works...not
real sure i trust them now. Any ideas now?