Looking for an explanation and advice on a problem with oven heat.

Purchased a Whirlpool Gold Accubake stove from a USED appliance store. This type of stove has the lower element below the oven floor. The owner of the USED shop has since died and the shop is not open anymore. I had spoken to him about the problem before he passed and he was quite willing to help, but died before I could make arrangements to have him look at the stove.
The problem is the oven.
1] It will preheat to what I set it to [[ie]350F or 400F], but it takes longer than the 5 minutes it goes into preheat. More like 15 - 20 minutes.
2] Once I put the food into the oven it stays at preset temp for about 1/2 hour, then the temp starts dropping off to under 300 F. If I increase the temp or raise the dish/pan up to a higher rack it just burns/dries out the top of the food.

What I have done.
I purchased a temperature guage to check these internal oven temps.
Removed the lower element and plugged it in outside of the stove and it checks out OK. [heats up]
Everything else seems to work on the stove, but I have not checked the self clean part yet.
I would like to know if anyone else has encountered this problem?
I cannot seem to find a fuse panel on this stove.
I wonder if I could install a lower element on the interior of the oven?

Thank You