My GE JBP23GS oven way overheats?

Recently I cleaned the self-cleaning oven on my GE JBP23GS. I followed the very brief instructions printed on the range top. Several days after cleaning, I tried to bake brownies. I set the oven to 325F and let it warm up for a couple of minutes before putting the pan in. After a couple of minutes (seriously, 2 or 3 minutes), it became apparent the brownies were incinerating. When I opened the door, it was like a blast furnace. I removed the charred brownie mix (sigh) and deduced that the oven was too hot.
I checked my O&M manual; read the self-cleaning pages. Tried again, this time following the O&M instructions to a T (main differences being: I set the clock [manual not digital], and used both the "stop time" and "delay start" knobs as specified in the manual).
After apparently successful cleaning cycle, waited until unit was cooled; then set "oven set" to bake and "oven temp" to 200F; tried again. Little cheapo oven thermometer registerd 300F after a few minutes; the 2 indicator lights ("oven cycle" and "oven on") were still on 5, then 10, then 15 minutes later. I'm assuming it was continuing to heat because it was darned hot when I opened that door!
Next test performed today: made sure both the "stop time" and "delay start" knobs were in popped out positions, and repeated previous steps. Same result.
I then thought Wow! maybe the thermostat needs to be adjusted. Followed the instructions in manual; oven still overheats.
I am at the end of my creativity - any suggestions?
Thanks!