Coolant temp jumping up and down

[littlej]

I have a 2012 KW T660 with a cummins ISX and my coolant temperature is jumping up to 220 or higher & then comes back down quickly. I changed the temp sensor & the thermostat and when I start it up the temperature went to over 200 almost immediately causing the stop engine light to come on and after idling it for a few minutes it's goes down for a few minutes then back up again. Anybody else experience this? Any help would be appreciated.

[Rawze]

The brass ring/turkon seal in the bottom of the thermostat housing will cause this too. Always change it when changing thermostat.