Yamaha DTXpress/DTXplorer/DTXtreme

It sounds like pickup of some interfering signal, although why it
should be worse outside is a puzzle.  Normally such problems occur
near mains cables.  While you have tried a different cable, have you
checked the Y splitter (or does the bar pad not need one?).  If it
isn't the cabling, I would suspect a bad connection in the bar pad
itself.  You could try two pads & a Y splitter to see if that also
gives the problem to eliminate the module as the cause of the problem.

Keith.