If the tool is sparking to the extent of burning out a pair of brushes that quickly chances are a couple of the bars on the commutator have been overheated and damaged. If the armature had been polished at the onset of the sparking problem it may have been possible to save, but, heavy sparking through several sets of brushes would have caused the armature to be damaged beyond repair and at this point need to be replaced.sorry.
Skil/ bosh worm drive saws are some of the best on the market and hold up to very extreme conditions. Originally designed for cutting heavy beams all day alot of guys started using them for stone and cement. Although they will hold up for a while in this enviroment the stone dust will eventually kill the bearings and make the saw start to spark.

Terry is on the right track here. It's more expensive than the brush replacement, but it looks like a new armature is most likely going to be the way to go.

Here's a link to the 5175 Armature in case you do decide to replace it. Both versions of the 5175 use the same armature, so you shouldn't have to worry about any compatibility issues.

Great info on the stone dust, by the way. Dry-cutting stone probably isn't a great idea for saws that aren't made specifically for it - even if these 5175s are pretty tough.

:) Thanks for the advice. I'll look at the armature real close and see if one of the lands are messed up. If it's still solid I'll see if I can put it on the lathe and true it up.