If you're doing straight cuts (or attempting to
) you could try clamping a fence to the table. This should help, and since that one never came with a fence and piece of straight stock, wood or metal would work.
Another thing to be aware of. The blade guides, whether they're friction blocks on lower models or actual live bearings on better models, are attached to the blade guard on the top side. If you're cutting a 1/4" piece have the blade guard set for about 1/2". If you have it set up real high the blade will lack support close to the work and be able to twist easily.
Play with the speed at which you're feeding it like Ken said. With plastics they can, and will, heat up. If they get too hot they can melt and clog the teeth in the blade reducing you're cutting ability and causing it to wander.