thermestat changed(gmc thermestat) and still not right

[molsongolden2006]

I changed my thermestat last nite bought a gmc thermestat and went to work this morning and the tempeture gauge still would go up pass 100
then back down in the normal range and when it does that it turns into
cold air? i topped off my antifreeze and tried to get the air out...what am i missing? could it be something else?

I was thinking there was an air lock in the lines so i took the cap off the radiator
and started my truck and let it idle for a bit and it seemed like the air came out a bit.....should i do it again..

can someone please help me out!!

Thanks