toby,
Are these the vinyl screens from DaLite that you're using? If so you may want to switch to one of they're fiberglass based screens. The High Power and the Silver Matte are fiberglass screens.
Those marks look to be caused from the rolling up of the screen too...
They seem to be evenly spaced. Did the DaLite rep talk about vinyl vs. the fiberglass? Call them up and ask them about this.
mech