Ahhhh, another typical case of the (well-publicized here) MH model's lack of tracking accuracy!
150 inches is more than the specifications on that machine clearly say --
Max Cutting Length
96 in
Anyway, I suggest this --- turn each of the knurled screws on the three pressure roller springs, but don't tighten them much, maybe a half-turn down force. I had an MH years ago, and that worked for me to correct some of the 'drift' but it never was 100% for long feeds, and I always had to allow for maybe an inch or so of shift (even if I had the vinyl pre-fed from the roll and it tracked fine when manually setting the alignment, once cutting it would still creep over a bit, so my designs which extended beyond 8 feet had to allow for that inevitable movement, and I would only utilize 22" on a 24" material).