How do I stabilize power to my PC from an erratic power supply?
You could use a voltage stabilizer, but it makes little sense to do so.

Almost all UPSs are specifically designed to work with dirty power sources.  The UPS working range is probably as good, if not better, than any comparable stabilizer in the same power range and price class.

It would be more cost effective to replace the UPS with a model which has a wider input range than to add a stabilizer.  This also means there are half as many things to fail.