High Engine Temps. and Check Engine

Hey, I have a big Problem.. I have a 97' Thunderbird 4.6 V8 and The Engine Temp. takez a while to warm up and then when itz warm it goes higher then it should be.. The check engine light has also came on now and my gas meter goes from half-way to quarter tank then back to half-way. The car iz running fine for everything otha then those problemz stated.. o by the way, im buying a new thermostat, upper and lower radiator hoses and new coolant tommrow so hopefully that helps, does anybody have any suggestionz of wat might else be the problem?
Thankz

Well, the tstat seems to be the cure for the first problem. Couldn't answer the second for you.

Thankz, i just found out that i needed a new 2nd Cat Converter and one of my 4 O2 senors.