You failed to mention what you have on the input, which can be a
factor--if you disconnect any input cables, short your input to ground
right at the amp and the unwanted stuff is still there, is is very
possible your unwanted signals are coming through the 5K gain pot and
its leads. Signals on these leads directly modulate the output. Try
this:

Replace the 5K pot with a resistor (say, 1K--the lower the R the
higher the gain) soldered right on the board. Disconnect the pot
wires completely. If that cures your problem consider eliminating
that gain control and just using the volume pot. Otherwise, you could
try shielding those wires, with the shield tied to signal ground,
though I'm not completely sure that would work. Use stereo shielded
cable with both wires in the same shield if possible. Twisting the
two wires together may help.

You can also try Mr. Masta's RF bypassing, only on the gain pot inputs
as well.

GL
Glenn