1997 Pontiac Firebird Engine Temp guage fluctuating

My 97 pontiac firebird/trans am was recently worked on by a friend mechanic. The issue was the coolant leaking due to bad radiator. Another issue was the fan - was told that it was modified by the previous owner to keep it always running and thus the temp never reached optimum temp 210 (middle mark).

Fast forward. He undid the modification and put a new (fan relay in). Now the temp goes up to 210 but takes a while. Then it will even go past 210 during times of idling to about 240. On the freeway it tends to go too low to 180 deg.

Any ideas about the temp fluctuation? I'm concerned about over heating as this has happened once since the repair. I've been trying to determine when the fan starts but can't pick up the sound when the car is on. PS the thermostat was taken out and tossed, no replacement.