Check Engine Temp. Codes

I have a 98 XJ sport 4.0, and have been fighting with check engine light problems for most of the summer. After replacing the O2 sensor and Engine coolant temp sensor the light has mostly stayed off until yesterday. What happend is I turned the jeep off at operating temp and started it again a few minutes later. The temperature guage shot up to about 240 and the idiot light came on. The guage then drops back down to normal, and after about a day-2 at the most the light shuts off again. Ive gotten a scan before and it gave me a ECT sensor temporary code the first time with these same symptoms, so im assumning its the same code. Ive cleaned my sensor, and will be doing so again after work, are there any other suggestions as to what could be causing these temp. codes even though all the parts are new? Ive checked for bad plug, faulty wiring, and to the naked eye all seems normal....