I'd try to avoid just throwing new parts at it at this point. Try to do all the free tests first.
I had a similar problem and went the route of 2 new thermostats, radiator cleaning (the shop said the inside was fine but they cleaned it anyway), fuel mixture and ignition adjustments (which was ok because those were free) but it still ran hot sometimes. In the end it was an errant temp. gauge.

The gauge would act normally in most instances but then go nuts during prolonged idling. It seemed like increasing the idle would cool it off just enough to bring the gauge back to normal. Ron's picture will be helpful but unfortunately I don't know if there is any way to "fine tune" the gauge. The area between 80 and 100 degrees C is where my was failing.

The way I narrowed it down to the gauge required the use of a 72 144 by;
1. Parking the two cars next to each other.
2. Remove the mechanical sender from the 122 and Install the temp sender from my 144 (electric gauge) into the 122.
3. I then ran a jumper wire from the 144 sender (now in the 122) to the pick up wire on the 144.
4. Connected jumper cables to connect the negative battery posts between the two cars.
5. Start the 122 so the temp starts coming up.
6. Turn the key to "run" on the 144...(DO NOT START or you will have a anti-freeze fountain) and watch the gauge and see if it acts normally. I think both gauges are similarly calibrated.
7. If this proceedure doesn't produce any temp after 5 minutes, shut the 122 down and check your connections. You don't want to overheat it in the event that it is not the gauge.

I found that the 122 was operating at normal temp. according to the 144 gauge. This was good enough evidence for me so I then purchased a new gauge.

I realize this required the use of another car, but you asked for ideas!