I have the PLV-60 and RP-91 and have never seen this. I always have my PLV-60 set to Auto.
Would you happen to have your Progressive setting to "Off"? When you are using non-progressive output from your RP-91, you should have your Progressive setting on the PLV-60 to "On". This is somewhat non-intuitive because the setting refers to the progressive scan mode on the projector, not the output of the DVD player. When you are using progressive output from your DVD player, the progressive setting in the menu is grayed out and is presumably set to "Off"
Also, there has been some discussion of horizontal instability using progressive out put on the RP-91 related to the frame rate. Supposedly, the best setting for this is Auto2 in the menu.