If you're certain that the battery and cables are providing good current the the starter, then the starter could certainly be on its way out. I haven't had to replace any Cummins starters, but I had the exact same problem on a Nissan car I used to have. I bought it used to commute in several years ago. After about a month, it started to turn over slow. I installed a new battery (1000CA) and cleaned up the terminals good. There was no change to the cranking speed. About a week later, I went into a store and when I came out, it wouldn't turn over at all. 1 Autozone starter changed in the parking lot later, it spun nearly fast enough to drive. Expecially if you're running a stock '92 starter, take it off and either upgrade it, or have a reputable shop do it.