Becuase of the irregularity of your prob. I'd suspect 1 of two things:

1-The thermostat is past it, sticking closed. Girlfriends '81 has brand new thermostat, rad, fan clutch, and w.pump, thing never runs above 1/2 on gauge no matter what. Volvo mech. may have skipped replacing it as brass t.stat body will mate to aluminum cylinder head and need to be "chizeled" out.

I went to a colder 87 deg. c t.stat on my 83 245 to help the car run cooler, original radiator is coated w/ aluminum deposits and is inefficient but doesn't leak so I can live with running the heater esp. as summer is over in OH. Also replaced mech. fan (clutch was freewheeling) w/ elect. one from boneyard, cheaper than a new fan clutch. Can really make the gauge drop toward C with judicious use.

2-Voltage reg. for gauge is bad, had this prob. w/ a 74 142. Gauges operate at weird 10 V, there's a transitor/chip behind the dash. But I think the fuel gauge would be affected too.

21/2--how's ignition timing? Could it be off and/or "jumping"?
My exp. stops at '83 unfortunately.

Frank