this started happening recently:

as im driving, my engine's heat gauge will slowly increase, and dip down almost into the red, just for a few seconds, and then go back to halfway and stay.

it will do it on the highway or stop and go traffic.
it wont do it again until a cold start.

any ideas?

 

You need a new thermostat.

 

I just had this exact same problem with mine, turns out my head gasket had a TINY leak which lead to a bubble in the coolant system. the thermostat due to its location will not open till the block gets hot enough to open it.

make sure the coolant system is FULL, cycle and check again. An air bubble anywhere in the coolant system will cause that problem because it will find its way up to the thermostat after you shut the engine off and it quits circulating, cost me a head when i didn't worry about it thinking it as long as needle came back, no problem. I was wrong

 

Yes, Or you need to burp your cooling system. Might as well replace the t-stat.

 

ok, thanks. i will replace the thermostat first, and see what happens.

 

oh, one more question... what thermostat does everyone recommend ? and what temp ?

 

what i would do is flush the the cooling system and replace the t-stat

 

I suggest an OEM dual stage t-stat from the Dealer. or a single stage OEM one. Aftermarket ones will work, but OEM are 97% the times better for our Yotas
190-195 temp is a good range.