Car running hot

Have you had the system pressure-tested to make sure it is not leaking? Is your coolant level staying full or is it falling? If you have had the system checked and it is ok, my next question would be are you well over 100k miles? Most of the time, not all of the time, if the cooling system checks out ok and holds pressure and the thermostat is not the issue, it is likely a leak around the head gaskets. One way to check is to change the oil. If it is regular, dirty oil then you can rule out head gaskets. If there's any water in the oil, or coolant in the oil, or if it appears to look like a "chocolate milkshake", that means your head gasket(s) are blown. This is a very, very expensive repair so hopefully you just have a leak in the system somewhere and a mechanic can trace it down for you using a leakdown and dye test. They will put the system under pressure, add dye, and then check the rubber hoses with a flash light to determine if there are any leaks and, if so, replace the hoses. Sorry you are having this trouble.