You may want to check the tracking and alignment on the cartridge. I had the same problem on the MMF-5 to find out it was a cartridge setting situation.
My symtoms were a bit different where as I would start to get distortion after reaching about midway through an album and more noticable on heavy passages. Proper alignment and tracking on the heavier side of specs (about 2 g on the Goldring) made things much better. The Goldring on the MMF-5 is a nice cart for the price but can be unforgiving on older, not well cared for vinyl.
Just a thought.