I usualy find out the center tab of the mogel fixture is overheated by a loose bulb I would suggest you buy an new bulb, if its not that you always have a spare bulb. I have replaced a few hundred bulbs and they last a real long time.
1 the base of the mogel is replaceable
2 make sure that all the electrical conection are tight WITH THE POWER OFF
3 make sure the ballast to the bulb cord does not exceed 15 feet
4 make sure the plug into the wall is not burnt or bad fitting.
still have a 30 year old ballast going strong
Ohms Ohms Ohms law always wins
keep your nuts tight, and do it with out shorts