this happened to me when one of the heating elements went out in a Smeg I owned. Somehow only having one of the two elements working made the other never shut off. Could be this, could be the thermostat, but in any case you're looking for an appliance repairman, not a cook!

yea, my oven did this when the thermostat went bad

No two ovens (gas or electric) are calibrated exactly the same way. But electric ranges are absolutely diabolical. Just got rid of mine. With electric the temp fluctuates and doesn't hold at a steady point, which is really frustrating because there really is no correct setting.

I think you might need a new thermostat.

Does it have a preheat setting? Maybe just make a time chart to figure out how long it takes to get about 25º F past the temperature you want (to account for cooling when you open the oven.) Otherwise consider foregoing the preheat, and just use the bake setting at the desired temperature, and wait 15 to 20 minutes before putting your dish in.

Unfortunately, I think this is more a question for an appliance repair person than a chef.

I'd start by calling the manufacturer's tech dept. or helpline - if needed, they can direct you to a local licensed service company. (The owner's manual or manufacturer's website might also have a troubleshooting guide - but this really sounds like something that should be looked at by a professional.)