Sounds like the sprinkler heads are a tad too low and that you have a sandy- sandy/loam yard.
A easier repair is to cut a 2" ring of grass around the sprinkler head , keep it clean and make sure the heads are exactly at grade so the pooling of the water from the spray doesn't collect sand onto the stem of the rotor. I've seen people use a 2" pipe to go around the canister to keep the dirt minimal inside. Looks a little weird up close but it does help
You can take the rotor apart ( Not the internal section of the stem) .. But simply unscrewing the canister, pulling back the stem and rinse it off , getting the grit off and stretching the spring some will help too.
Also the 42sa is just a crap head to begin with... you get what you pay for right? Upgrade to a better head like the Rain Bird 5006 stainless steel sam, it'll take more then just some grit to break down this baby