I for one would convert the engine to the Cleveland design. There was a lot of money spent by Ford on engineering and I don't like to reinvent the wheel. The Cleveland design is not a faulty design. It is just different.
As far as the rise and fall there could be something happening in the engine such as a bearing but what you described to me I consider an early warning that the theromostat is on the way out.
It sounds like it is sticking partially open when it should be fully open. I personally have often ignored that warning only to have the thermostat totally lock up and over heat the engine.
A thermostat is a mechanically "fusible link". It is the cheapest part of the engine. Just change it and see if you have the same symptoms.
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