Temperature gauge rises when idling on 1996 SL2

[albundylives]

I've replaced the temp sensor unit, and am assuming I need to replace the thermostat. When I idle, the temp goes up to over half way, then when the fan kicks on, it goes down a bit. When driving and the wind is hitting the radiator, it operates a normal temps. The only thing that keeps me from thinking a bad thermo is it doesn't overheat, just rises. Too much thinking on my part and just replace the thermo?

Thanks in advance

Polk High's Al Bundy