I just ordered a Ranco from reptilebasics.com.

After fooling with the rheostat for the rest of today I see what everyone is saying about them. I though the temps had become stable around 90, and an hour later I went back in and they were 96.5! I dialed it all the way to low and left it alone until now. I just checked it, and going from having it set between low and medium, and bumping it all the way down to low, I'm still at 95.6. From what I've read on here 95 is kind of the limit on how high the temps should be on my warm side. I just don't trust that thing. Hopefully I can return the rheostat and get my money back for it.

Other good news is I put the foil on the screen top and my humidity has been stable at 60% all day even in this Texas heat!!