As i mentioned before it could in fact be the stat stuck open.
Do this,when motor is cold from sitting over night start it up with read cap off and simply rev the motor a little to see if the coolent is circulationg like it normally would when its warmed up.
If the coolent is circulating as soon as you fire it up cold that would tell you if the stat is stuck open.
Also at the same time use the temp gun on stat housing to see if it ever hit anything close to 180 deg f.
It almost has to be that becasue even isf the cooling system was too efficient for your setup the temp would get close to 180 then drop as the the stat opens but when things coole down below 180 the stat would close off some again to reheat the coolent back to 180 and the temp would rise untill the 180 mark then drop again going back & for like that but never stabilizing at 180 because the cooling system was doing to good of a job which is unlikely but you never know.
AAAAAAHHHHHHHHHH,what about this.
You stated temp goes to approx 155,maybe you accidentially installed a 160 deg stat of it was packeged wrong with 160 stat in 180 stat box.
If the coolent doesnt circulate with motor cold then shut it down before it gets hot and remove the stat and look on it to see what temp is stamped in the stat body to see if it says 160 or 180.
What ever it state replace it ,thats for sure but i bet it may just be a 160 stat and you think its a 180 stat.
Scott
