Temp gauge up and down

[grooveman]

I recently replaced the radiator with a 3 core and since have noticed the temperature climbs up to about 220 before the t stat opens, then goes way down, then back up to a little over 210 then back down, etc.. back and forth over and over. The sweep gradually gets smaller then it sort of stabilizes at around 195.. I think it may be time for a new t stat. I had a problem with corrosion in the system (previous owner not me) which is why I replaced the radiator. Any ideas on this? If its the t stat should I go with the 195 or 180? It gets real cold here in winter..
Thanks

 

[jhiggins]

Go with a 180 and it sounds like the thermostat

 

[jeepinstien]

Go with the 180 in the warmer months and 190 in the colder months to keep your heater toastie.