overheating update

[ocbaud]

okay, yesterday i had my dads new radiator cap on and i changed the tstat to a 160 model and also drilled 2 small holes in it, in case there is any air pockets that need out. MOST of the time it will run really cool. the heater works well, but not as warm as it was before with the 180 tstat.
every once in awhile(either on the highway, geting off the highway, on the street doing 50 or 5) it will steadly go up to 260degrees. when its that hot, the heater is still blowing the same amount of heat as it was when it was running at normal temp. after maybe 5 to 10 or so seconds, it will QUICKLY drop back down to operating temp. i would say it takes about 2 secs max for it to go from 260 back down to 160 or so. after i got home, the overflow tank didnt have any more coolant in it than normally. i never smelled antifreeze and it just didnt act like it was really overheating.

could a faulty sender cause this?