Mitsubishi W65713 has wavy picture

Hi all.  It's been a while since I posted to this forum when Larry Dillon helped me with the "blinking green light of death on my Mitsubishi W65713 a few years ago, when he told me exactly how to replace the capacitors (about a $3 fix for me plus a case of beer ... he saved me a lot, and I am forever grateful).  Ever since the fix he helped me with, the TV had been running perfectly.  Recently, however, I have come across a different problem.  When I first turn the TV on, I get a clear picture as usual, but after about 15 or so minutes the picture suddenly starts "waving" so that it seems like I'm looking through water (vertical sinuous, jiggling "S" waves, no black lines or anything, just the picture itself distorting).  It will stay that way, sometimes worsening a little bit, sometimes getting better, and sometimes evening snapping back to a perfect picture for awhile.  

I have the service manual for this unit, but I need to know what to fix here?  Does anybody know what is causing this and how do I fix it? Any help will be appreciated!