I am pretty sure this is a software problem, not a switch problem. The T5 is the Ford Edsel
of the Palm range, with many unsolved bugs. I fixed this problem by using shortcut for ON and OFF.
Use one of the 4 application buttons. The default house button opens the Palm at the menu, but you can change any of the buttons in
Preferences > Personal Buttons
to open whatever application is convenient.
Preferences > Other Offstroke
check Enabled, and Pen Stroke or Screen Tap (or both), and then select the gesture that will turn off the Palm. I use "Screen Tap in top left corner" to turn off.
It's not a real solution, just a workaround. I also flushed and reset the T5 and reloaded from backup, and the problem went away for a while, then came back. It gradually takes longer and longer to switch on and off. There is some bug in the OS, but Palm doesn't seem to care about it.