Steve
I had the same issues with my Samsung RF18. I replaced the defrost element thinking that it was not defrosting correctly. No Joy, same problem continued. The next time I ordered all new parts to go with a new defrost element (defrost sensor element and thermo-fuse). When I took it apart the second time I carefully examined all the parts. The thermo-fuse is encased in a clear plastic enclosure and heat-sealed closed. The seal had failed on mine and it was full of water (problem found). The new defrost element and thermo-fuse solved my problem. I also installed the new clip on the defrost element that extends down the drain tube. So, for the last two years, no problems.
Richard