Electric oven temperature issue?

I cannot get an accurate temperature from my newish electric oven, and it's inconsistent in it's inaccuracies. For example, one day I'll set it to 350 and it think's it's reached that when it's really only 275; another day I will have to set it to 500 to get to 375. Any ideas? The last repairman I had out told me (basically) that I was imagining it. I may be nuts, but I'm not that nuts!