Quoted from Leosac:
Every ball that rolls over a "base hit" doesn't matter double, triple or homerun in the outlane draining the ball, it scores 5 runs.
Do you mean that either outlane rollover switch scores 5 runs? Looking at the photo on IPDB (http://www.ipdb.org/showpic.pl?id=1816&picno=14403&zoom=1) both outlanes are Home Runs, and looking at the schematic those are the only two Home Run rollovers.
If that's the case, I'll assume that the H/Home Run relay closes and the F/Run relay fires 5 times when either of those outlane switches closes. That can happen if there's a problem with the Hit Unit, which I think is a stepper:
Playball schematic segment (resized).jpg
If the Hit Unit is stuck in the 3rd position you'd see 5 runs for each Home Run. See if the Hit Unit advance solenoid fires when you roll over the Home Run switch. If it does, check that the stepper's wiper disk is advancing each time. It might be as simple as having to tighten the locking bolt that holds the wiper on the shaft.