i know there are threads on here concerning this issue. but my issue is slightly different.

i was out in the shop and was installing my new throttle body spacer. i had just finished and wanted to go and try it out. (not really expecting crazy noticeable difference.) anyways i was just pulling in the driveway, and i noticed my temp gauge was at 160. and i know it was at 210 not 15 minutes before that. anyways i stopped and looked under the hood. everything was normal. i went home and let it sit for quite a while. came back out and started it. it went up to about half way between 210 and 160. it stayed for maybe 5 sec.and went back to 160. i think it might be the ECS. but still confused as to why it what read correctly then quit and not work.
Any suggestions?

i read up on similar situations but nothing quite like this. truck runs fine, its not shutting me down, AC works.