I guess this thread has wandered a bit on cooling for transaxle cars. Since you have upgraded your cooling system with the new fans, you should be running pretty steady with some normal fluctuations. Funny thing is that you say it was fine for part of a recent drive (presuming A/C off), and you know with the fans on you have sufficient capacity for most normal driving. I would lean towards a sticky thermostat if purging air does not help. Does your car seem to take excessively long to warm up? I know, harder to see that in the summer/early fall. Make sure you have good, clean connections and grounds on the temp sensors and gauge. Check if you have an old hose partially collapsed or with an obstruction. Bleed the air out of the system. I am assuming all new and good coolant.
A bad cap or crack in the tank will bleed pressure, lowering the coolant boiling point and letting air bleed back into the system. Check the tank. If the crack is on the top you may not have lost any or just a little coolant. I have had very bad experience with the aftermarket tanks. Mario @ Eurotec has been able to scrounge up good used OEM tanks that last a long time but they are getting scarce. I now have an aluminum tank ready to go, it looks super nice. You can find the thread for the aluminum tank here on the BB. I think I saw a recent note that the alu tanks are still available from the last batch manufactured.
side note: check out provamotorsports.com. I'll send you a PM
'82 gtv6 3.0, '86 gtv6 3.0, '18 Stelvio TI