1. Not sure,
2. There are a number of things to check first before you jump into the guides.
My first thought is to check the alignment of the wheels. Grab a straight edge and lay it across the top and bottom wheels, look for gaps between the edge and the wheel. You want them to be co-planer, you may need to shim them.
Second, inspect the tires and make sure they are of good repair with a noticeable crown.
The blade should stay on the wheels with the blade guides completely removed, the guides only function is to aid when cutting. The above sounds like a set up problem, not a guide problem.
Adam