Transmission downshifts and hesitations

[seb001]

I have a 93 Caravan, with a 3.0L engine.

A few weeks ago, I changed the oil and the thermostat. The old one was always opened and the engine temperature was fluctuating up and down. Right after, everything was fine, the temperature was steady.
Two weeks after that, I was on the highway and suddently, the transmission shifted down, then back up. It kept doing this. If I pressed on the gas, it would respond properly. It was only when the car was at a steady speed. I made it to my destination and about three hours later I was back on the highway. It started to shift downand up again. I found the sweet spot and managed the 1 hour drive home. A couple of stop sign from home, the transmission would not shift up until the RPM were high. The next day, the car would rough idle and was hard to start. Cleaned the spark plugs and lowered the fluid level in the transmission. I replaced it (tranny) 16 months ago and noticed that the fluid level was too high but it was running great so I did not worry.
I suspect that the oil got hot (more than it usually did with the old thermostat) and eventually foamed...

Everything was good for about a 10 days and then I noticed some hesitation when accelerating. The sparkplug wires were old so I changed them along with the distributor cap. I also changed the fuel filter.

Two days ago, I again noticed that the hesitations. Is't hard to tell if the transmission or the engine are causing the problem.

Today it's at the mechanics. Of course when he drove it, he did not notice any hesitations. He said the these engines tend to have lots of buildup on the injector and valves. I agreed to have the injector cleaned (in place). Apparently it will also clean the valves.

I mentioned that maybe it was the timing belt but he said that if that was the case it would probably be affecting the car at idle. (idle seems to be fine).

Any ideas?

Thanks