[Roadsters] Roadster OT: Pathfinder (VG30E) temp gauge wonky

[O'Farrell, Fergus P (AS)]

Trying to help out a family friend to whom I owe numerous favors.
Rode in his ~86 Pathfinder (VG30E/5spd) admittedly in freezing weather
(10-20F), and it seemed to me that the thermostat has gone wonky, but need
critique of the diagnosis.

On start-up the temp gauge stays on dead cold, for quite a while (longer than
it should), defroster blows cold, so it is truly cold.
Then the begins to rise, and then cycles up/down with the engine speed, (mind
you we only got up to 25-35mph, but it was ~15 degreesF) as if the revving is
injecting heat, but the temp gauge is responding almost as quickly as the
tach.  (possible parallel elec issue?)  Starts to blow slightly warmer air out
of the defroster, but never really gets HOT.

I am thinking the thermostat is getting crusty/clogged, so is slow to respond
initially, then is snapping open and staying crustily stuck in that position.
(but wouldn't the temp gauge be reading radiator output water temp, and thus
be reacting somewhat slowly to engine revs,  not reading radiator input water
temp and thus reacting quickly to heat inputs?)

Perplexed, and trying to pass on good advice before someone digs in to an
"dry, as yet undisturbed, but with 130k+ miles" engine.

Thanks, and happy holidays,
Fergus O
(thankfully back to a warmer climate)