Vehicle: 1997 Buick Ultra (Vin 1 engine), 100,000 miles.

Background:
Before any work was performed, the car would warm up from ambient temperature (e.g an ambient temperature of 70 F) to 192 F to 194 F and immediately stabilize within that range. The digital temperature readout never got above 218 degrees, even under a heavy load.

Work performed:
1) Leaking intake manifold gasket replaced (thank you GM engineering).
2) A change of coolant
From - Prestone Dex-cool 55% / 45% distilled water
To - Prestone Dex-cool 55% / 45% distilled water
3) Iridium spark plugs replaced with new AC Delco Iridium spark plugs.
4) Transmission filter and fluid change.

After the work was performed:
When the car warmed up ( e.g. from an ambient temperature of 70 F), the digital readout showed the coolant temperature would rise to 210 F, and would then fall to 189 F. within a 10 - 12 second period. It would then slowly stabilized between 197 F to 201 F in about a mile.

I then had the old AC Delco thermostat (195 F) changed with a new AC Delco thermostat (195 F).

Same symptoms but with coolant rise to 207 F falling to 187 within a 10 - 12 second period and stabilizing at 196 F to 198 F in about a mile.

I live in a mountainous area where I frequently am driving 7% - 8% grades for two miles. The temperature readout now climbs to the low 120's - something it's never done before.

The probability of two faulty thermostats (one being brand new) is extreemly low.

So what possibly happened as a result of the work performed?
Or, what else could be wrong?

Thank you.