ok i think you just need to pull the gas pedal back a little and not get on it all the time that eats your gas up really quick. no your temp wont effect that and no its not hard to replace a thermostat. you could have a bad fuel filter, o2 sensor, injectors etc.... my 90 integra runs hot when its just sitting there also havent really looked into it that much cause it only does it if its sitting there running for like 30 minutes. my first guess is there is water bubbles in your radiator or you thermostat is bad. but it could also be a leak some where do you have to fill your radiator up