I had exactly the same problem with my Evoke-2XT.
I solved it by prodding the tuner knob with my finger for some considerable time with the radio turned off. It took quite a number of prods but it cured the problem so it could have been that some deposits had formed on the push switch and the continuoue prodding cleaned it off. Its worked ever since.
Failing that working contact Pure's support who I've always found to be excellent.
Stephen, April 2010