Ah the SP. That was the first machine I learned on years and years ago. Unfortunately, I cannot remember anything about that thing...Since no one has responded.
Clean the crap out of the printer. (heads, encoder strip etc..) Also, don't let your machine pull the roll. Always make sure your material is forward-ed out because that small amount of pressure can create banding. Static Electricity could be affecting it.
Ah, I wish I was more help. Sorry mate. I assume you ran all the appropriate test prints? Good Luck. Here's to hoping you get to upgrade that old thing.