Frigidaire upright freezer Model #FFU17C3AW1 Not keeping cold

I have owned this upright freezer since new. mfg date 06/2002. Manual thaw. Freezer is not staying cold enough to harden Icecream. Just a couple weeks ago, we took it in to a local repair shop. They had it unpluged for 3 days. Plugged it in, for 4 days. had special internal tempuratue gauge. Said ran just fine. Got real cold quick and stayed cold for the 4 days they had it. I got it back home, plugged it in the next day to let oil go back into compressor, ect. put food back in and worked AWESOME!!! Icecream stayed hard. 1 1/2 weeks later, Icecream was soft. I thought to unplug freezer. Two days later Icecream was solid again. GREAT!! I got it fixed....right. week later Ice cream is cold again. I presume the problem is the t-stat. Is that the problem and if so, is the part # to replace 216788100? Where to buy t-stat. shipping to 84660

Thanks