horizontal lines in projected picture

So for the 1 upness of it I installed a dvd player and a Sharp Notevision XR-1S dlp projector (connected w/a s-video cable) in my van.(I get movies about 4' wide)Unfortunatly I'm having some problems with horizontal lines rolling up the screen when I play a movie. I am powering the projector and dvd player with a Coleman 1200w (2400w peak) power inverter wired to a deep cell battery that is wired to my alternator. Nothing else is running off the inverter. When I pull the set-up inside everything works fine. But, back out in the van the horizontal lines appear again. I have tried with the engine running and with it off. It doesn't seem to matter. The only thing I can think of is the inverter is using/putting out a modified sine wave (what ever that means). If this is the cause is there a filter I can get to fix the problem? Or maybe I can adjust the dvd player or projector like you used to do for the tracking on vcrs. I appreciate any input you may have.