I've had the same issues when using any driver setting that indicated non-paper substrates. Have you tried just telling it to use the correct GSM paper? Basically tricking the machine into thinking it's just a thick cover sheet. You may have to tweak the image quality related settings to avoid pitting or thin laydown, but this tactic has always fixed my similar issues.
Related note: I gave up on lasers for my screen positives years ago and went to inkjets for more consistent results. There are several 13x19 inkjets for US$150-ish which are far simpler to use for this application. Check out Casey's Film (about the same cost as laser positives).
Good luck!
"I'm gonna need to see more math I don't understand to believe all this"