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If it's electric, the heating element may be going out. Heating elements aren't really hard to change, if it's one of the normal styles with the element just sitting on the oven bottom. It could be a switch or thermostat, though, if by "seems to be preheating" you mean it starts to heat and then indicates it is heated. I shocked the bejeezus out of myself once playing around with switches on an oven. And it turned out to be the heating element after all. :)
If it's a gas oven, I don't know how to fix it, but my old one used to do that all the time. It would heat, I would start cooking, and the darned thing would just cut off. Actually, it would just not come back on when the temp dropped the way it was supposed to. Never could fix it.
Probably didn't help you any, but isn't it nice to know someone shares your pain? :)
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