Mitsubishi pixels dropping in 480p

I have a Mitsubishi WS65909 diamond series that about a year ago I successfully replaced the  convergence IC and fuses thanks to information from this forum.  About 3 months ago while watching a DVD I see a pixel is out then another then another and the longer I watch the more pixels I lose.  I do a hard reset on the set and the pixels come back.  This only happens when I am watching 480p material.  If I am on the cable box that delivers 1080i I am good no pixels drop.  This set still delivers a good quality picture and I was confident after I replaced the convergence chip because the picture was amd still is very good.

Any idea what could be causing this issue? 

Thanks in advance.