hp laserjet 5si

sometimes the problem is that the troughput of paper doesn't keep steady. At the top of yr printer the paper is pushed though a wheel. On the wheels are little springs. One of the springs must have been less rigged than the others. try to replace some springs for better troughput.

Good luck

Is it folding inside the printer or outside?

I recommend that you clean all the printer rollers and make sure that there's no dust gunked up against the rollers either.

If the paper folds outside the printer, then there may be a temperature thing you need to look at. If the printer is in a pretty damp/cool place and the paper is gone through the fuser, then that can cause some folding/curling problems as well.