Started losing acceleration about 5 months ago.

Started losing acceleration about 5 months ago, changed fuel filter and did tune up with no change in the performance. Took it to a reputable transmission shop to have the Tranny flushed and they said the tranny was bad. The van sat for several months while we weighed our options finally deciding to get it replaced. We got it back a couple days ago and it is still having a problem accelerating. Now though, it runs fine for the first mile or two, then about mile 3 the oil pressure light comes on and it starts running very poorly with little or no acceleration. Hooked a guage to it, it starts out cold at 50 to 60 psi, sounds and runs ok. Over a few minutes of driving it drops to 10 psi (before I shut it down) it sounds bad (knocking and pinging) and the acceleration gets worse and worse. What could be causing this and what do I test next to rule things out. I am not getting any codes when I plug it in.