Hello all!!!
I wish I found this site six months ago! Here is my problem.
I have a 2 pump spa with 2 diverter valves. The valve is a standard two inch valve in which the water is forced through the bottom and diverted to the sides by turning the valve on the top. They are very common and look like this: http://www.spapartsdepot.com/cgi-bin/cshop.exe/spaparts/caldera-valves.tam
One valve top kept popping off so I had to crane the spa out of the pit to try and fix it since the sides were not accessable. One had only a few threads poping up so I had to pull the valve housing up through the top of the spa. I tested it while it was out and the other side popped off. Why? The valve and top threads were good. I bought all new valve tops and valves like the one's at the top of my link with there magic lube. It seemed to work so I craned it back in. The second day the valve tops popped off again. The threads seem fine but the valve housing almost seems like it shrank. Has anyone heard of this? Is there a solution? I hate this thing!
John..

About the only thing that will cause an otherwise good diverter valve to pop like that, is high back pressure. If you have adjustable jets, make sure you don't have many of them at a reduced flow rate or off alltogether.

All the jets are open and water comes out. It has got worse. It will even pop off when in low speed. The whole way the valves are designed on spas are bad when the water flows straight into a pop screw off cap. The only next thing I can do is cut it out and eliminate the diverter.
John..

When you were fixing them, did you replace the housings? You might want to look into that, since plastic threads, when they start to strip, still look good. They just won't have as deep of cuts for the threads and that makes it easy for them to slip past the retaining nut. Could try to glue the nut to the housing as a last resort before replacing.