oven overheating & burning cakes?

I have a Concept Solar Plus cooker that has been great for many years in every respect. I do not use it a lot now being elderly, disabled & with a very efficient microwave but I do use it for a favourite rich fruit cake recipe that takes 2½ hrs at 120 degrees.

Suddenly it has started burning this cake to a cinder. The first time I left it for full time as usual to find house full of smoke & cake black & ruined. Then I tried a cake recipe that takes 25 mins at 200 degrees. Perfect. So thinking the cooker had had a brainstorm I tried the big cake again, with same result, that is burned & black after only 1½ hrs at 120.

I have just checked it again with empty oven, put it on for 1 hr at 120 - fan worked & temperature light went out OK after short interval, so I sent it on for another hour then checked after 30 mins to find light refusing to go off unless oven switched off completely, & element glowing red hot, with fan still working.

Any ideas & is the cooker too old to be repaired anyway?