SOunds a lot like negative pop like some have suggested. The wonders of heat expansion. I never believed it until I saw it happen and set out on a mission to figure out what to do about it. The different levels of sharpness on the edges tells a lot.
I am poor as well and was using darkroom time at the community college. The enlargers were the same as yours if I remember correctly. The looser a neg is held in the carrier the larger the pop. you have to tighten things up so the neg is really held tight with no room to shift. In my opinion if you can shift position of the neg when it is held in the enlarger things are too loose and negative pop will be bad.
I removed the annoying spring that pops open the carrier a little when you take it out of the enlarger. This allowed the carrier to lie flat on top of the neg. Then I made sure the enlarger clamped tight as hell on the carrier. The condensor weight is what is holding the carrier in place but it was not aligned so it layed flat across the carrier. Higher in the back by a smidge. If I remember corectly this was solved with brute force.
The problem was not totally solved but the smaller aperatures took care of the rest. To totally get rid of the problem you will need a glass carrier.
Good luck, I do not think new lenses will solve the problem.