I have two more options it may be mainly because I am hoping its not the optispark. One of the battery terminals is stripped out so that may cause hesitation while driving. Also, when I bought the car, the guy said he just had NGK spark plugs put in. If he put in the wrong ones, ones that run hot, that could cause major hesitation and stalling when it hits operating temperature.

Also, how hard is it to change the opti myself?

 

Te opti isn't hard it's just time consuming. I'm thinking it's your opti since it happened after you changed the t stat. You might have spilt some antifreeze on it which will cause it to go bad. Try taking your car put for a short rip them take out your plugs. They should be light brown. If the are wet then you have weak or no spark in that cylinder.

 

I put a towel all around the water pump when I was changing the thermostat to prevent antifreeze from getting anywhere. I sprayed the opti with some WD40 to try and draw out any moisture that is in there. Hopefully it isn't the opti.