06 mazda 3 heat issues.
 

So I got this 06 3 150,399 miles on it. I only had heat while in the throttle. I am a mechanic at a local shop,So I know a bit.Parts are cheap enough,Dont know the history So I went ahead and did the water pump and thermostat and housing I also flushed the heater core for the heck of it. The cooling system is clean as can be no signs of ever having stop leak or anything in it.
Its better But if I idle to long it most definitely Still cools off. I've been driving this thing for about a month now and it got a little warm this morning about 2/3 of the way but that was before I changed everything out. other than the temp has stayed under half way, To me in a decent spot I removed the thermostat from the housing and I couldn't open it at all So i am going to assume it was stuck closed, My lower rad hose is still cold even after changing everything.
I dont have any reason as of now to suspect a leaky head gasket I dont see any air bubbling up with the rad cap off.Runs like a dream 100% off the way other than like every 4 or 5 drives the tranni goes into safe mode for about .5 second if its in auto mode its downshifts to 3rd I again assume from safe mode. But literally like a half a second it shuts the at light off and switches gears. The fluid is clean It shifts perfect 100% of the time leading me to believe a computer control issue. I am wondering if its tied to the temperature. I know some vehicles wont shift to overdrive and such until operating temps ...
Any one got any ideas I should look at?