Hot Tub Heater Stays On..

[CobbCJ7]

I have a Catalina 7 person spa with a Balboa Cat-200-DCAJ  #51677-05. I got this whole thing for free and it's been a pain in the neck ever since. 

The only problem I have had so far is that the heater element never shuts off. It will climb all the way to 110deg or more. I've tried setting the temp level down at 90 and it blows right by that. I have burnt up so many elements now. What could be the culprit?? One of the Temp sensors??

Also, if the spa pump shuts down, which for some reason it does on its own. Or if I put it on the Economy mode were the pumps run just a few times a day, that heater element stays on and burns itself up. 

I don't have the money to go out and buy a $1,400 replacement control box so I was hoping for some suggestions. 

P.S. I hate to admit that I have smaller gauge wires than what is listed for the tub. It calls for 6 gauge and I am running 10 gauge wires. But, I only have one working pump currently so I don't see the need for the expensive bigger gauge until I get the pump fixed. The reason I added this is to see if for some reason the smaller gauge would affect the mainboard in a way that would not shut off the heater element. I assume not but thought I would add it.

Thanks and any help is much appreciated!!