I got an 85 300D a few months ago. 230,000 miles, nice shape, runs great. Needed typical minor work like tune-up and such. Been extra careful inspecting everything seeing as I got it from a little old lady and judging by the repair paperwork she gave me, she'd been getting taken advantage of by her mechanic. (Billed for 5 Bosch glow plugs, got 4 autolites and they didn't try replacing the rear one. Billed for 4 calipers and all pads, got 2 new calipers but brake pads were almost metal on metal, etc). Also, the vacuum lines to the locks and such seem to have been cut and plugged off, but that doesn't bother me. It's been running, driving and shifting perfectly up until a few days ago.

So I've changed all fluids, glow plugs, got everything working that didn't and have been driving it about a month. Everything was excellent. Trans fluid was at the correct level, looked nice, didn't smell burnt, so I left it alone for the time being. The other day while driving I noticed a bit of a vibration like a wheel out of balance and a sensation of being "held back" like my brakes were hanging up and the car was fighting it. Almost like tapping the brake pedal while driving because it would come and go.

Acceleration is no good now, it goes into 3rd fine, but acts as if I'm holding the brakes. It gets worse as I drive, it will rev to 3500 and stay in 3rd. I can let off the gas and then it will clunk into 4th and that will feel like driving a manual in the highest gear at 20mph. Can't increase or maintain speed. The smallest press of the accelerator then drops it back into 3rd. Oddly it doesn't feel like it's slipping. It's even like the car is actively slowing down when I try to accelerate. Like it's fighting something, or trying to pull something far too heavy. Like I have a semi tied to my back bumper. I can put it in neutral and it rolls along just fine though. Everything spins freely, nothing is dragging or hanging up. Every now and then it would act like it might snap out of it and would start picking up speed, only to start slowing again and worse.

I was on my way back from a 50 mile trip and by the time I got home I could barely do 35mph. Trans fluid looked great still, no shavings or discoloration but I drained the pan and torque converter, and changed the fluid and filter and it had no effect. Acceleration is slow, acts like something is actively holding the car back and slowing it down, if I let off the gas it shifts to 4th but immediately drops back to third if I push the gas as if I'm going up a steep hill. Engine runs like a dream, all fluids and filters are new, everything greaseable is greased. This is a sudden thing that happened during one drive and now seems like it's here to stay. Has anyone had anything like this happen? I've looked but it seems everyone else mostly has normal slipping or typical vacuum issues.

I have two parts cars with good transmissions, and while I'd rather not, I could replace it. Problem is the other cars are an '83 and an '84 an I've heard they won't interchange. Any help or advice would be definitely appreciated, because I'm all out of ideas!