Purchased an 84 300D about two weeks ago. The first time I drove it, I had trouble getting over 40 mph. After about 100 miles, I was cruising at 65 and 70. Three days later, it slowed down again, and I had it towed back home. I just added two quarts of Trans-X, after draining two quarts of tranny fluid. Drove it about 15 miles, which was a chore as, now, I can't get it over about 20 to 25. Could it be something other than the tranny? It seems to shift, and then bogs down. I can floor the accelerator, and it doesn't seem to matter. I switched to neutral, and the engine is definitely reving. Any ideas?