I second getting an oven thermometer - I think mine cost less than $5, and I found out my oven runs consistently 25 degrees hot. So I turn it down 25 degrees lower than the recipe says, and everything is fine. Now, if you find your oven is not off by any consistent number, or that it's off by a huge margin, then I'd call the repair guy or look into buying a new one. Depends on your budget, I guess.

IMHO, there is nothing worse than an unreliable oven. The first thing i did when we purchased this home was to buy an oven thermometer and check the oven, even though it was a recently-purchased stove by the previous owner. Luckily, the oven temperature was exactly right. Had it not been, I would have checked the warranty, but whether in or out of warranty, a repairman would have been called and the oven fixed or the stove replaced, if at all financially feasible.

I have had 3 second hand electric stoves in 15 years. A new one is not in the budget. With each one I have had to adjust my baking temps and times a bit.

I do have an oven thermometer and my current oven is 25 degrees hot. I adjust the dial to be at the right temp.

I have always rotated what ever was in the oven. Even when we had a convection oven that moved the hot air around.

It's a far-out possibility but are the oven off-on switch and temperature dial separate? The gauge might have been popped off for cleaning and replaced incorrectly. If gas, the thermocouple may need replaced.....the stove wouldn't know when the desired temp. was achieved and just keep heating.