This is my first time asking for help on GardenWeb, so please be patient with me!

I have a Frigidaire (yuck, I know, but that's what I'm stuck with for now) range/oven, about 4-5 years old, that was working fine until last fall. All of a sudden, it would take over an hour to cook a pan of brownies even though the temperature was set correctly. My friend suggested that I buy an oven thermometer, so I did, and it turns out that sometimes the temperature cycles correctly (+/- 20 degrees of desired temperature throughout baking period), and sometimes the oven never even reaches the desired temperature--last night, I baked cookies (preheated for 30 minutes), got tired of waiting for it to reach 350, and ended up cooking the cookies longer because it wouldn't go past 310.

Even more irritating is the fact that when I preheat the oven, it will beep to let me know that it's good to go, but most of the time the oven is anywhere from 20 to 50 degrees below the set temperature.

Is this a cheap "reset" issue--my garbage disposal has a little button that I can push to reset things... do ovens come with this feature? Do I need a new computer board type of thing? We did that with the old dishwasher and ended up chucking the thing because once the boards on appliances go, they are usually at the ends of their lives (in my experience, anyway).

Please help--I am not in a position to buy a fancy new oven, and I am obviously not technically proficient with appliances.