Strange fault in Zenith radio!

[Hemingray]

This one is entirely new to me, this just appeared awhile ago while running my Zenith 7G04. It played perfectly fine for about an hour or so (feeding it signal from my XM Xpress receiver). Out of nowhere, the radio begins to spew forth with loud bursts of static. Disconnecting my Xpress from it and hooking it to the outside antenna, EVERY STATION it pulls in is jam-loaded with these static bursts. Any clue what's suddenly started this up? tapping the radio doesn't cause it to go away, ruling out a possibly faulty tube connection. The only tubes that were ever changed in this radio is the 19T8 and 35B5. All of it's others checked out fine when I last worked on it.

Forgot to add, this is the second time it's done this. The first time was with the radio tuned to a local station.