2000 Dodge Durango oil pressure

Oil pressure dropped to 0. If we let it idle would build back up but after driving just a little bit would drop again. Replaced oil pressure sensor, rods and main bearings, oil pump, timing chains, flushed engine several times, took air compressor and blew out lines. Also removed the cover on the right side and watched to make sure oil was pumping, seemed to be pumping fine. Still no oil pressure. We also tried putting mechanical gauge which still read 0. Called mechanic, who told us that he has never heard of anyone doing all that, that they just replace engine. We took it to the mechanic. After two weeks there he called and said it was fixed. On the way home, about 18 miles it started the same thing. When oil pressure drops the engine knock. On the invoice from mechanic it says " removed timing chain cover and put vehicle back in time and flushed engine and added engine additive and replaced oil pressure sensor." We payed $600. We called mechanic and told him still doing same thing. He said he drove it around alot and it was fine, and we need to replace engine and he would give us deal on replacing engine. What in engine is actually broke? Did we miss doing something that would fix problem. We have read alot about engine sludge, if this is the problem how do we fix it. We did regular oil changes. We would like to at least get it fixed enough to trade in.