I will not argue with Casey's point, but I will interject that my thinking was more along the lines of Flexwatt being an after market product. Meaning, that you, or someone other than a manufacturer has to make a connection on the flexwatt. If the contact points are not goo, and one side of the flexwatt has a poor connection, the power could be irregular to the strip of Flexwatt.
With that said, the cheapest route to possible problem resolution might be to buy an UTH. However, Casey brings up a good point, in that buying another rheostat is not expensive either. Troubleshooting those two devices would be step one in my book.