Different colored vertical lines on BACK of pages

[AkamaiRBS]

I have over 40 HP LaserJets and can figure out most issues and replace parts as needed. I have 2 low-miles (about 45k) Color LaserJet 4700dn models and both are placing different colored vertical lines ON THE BACK of single-side printed documents. It doesn't matter the type of paper (20lb or cardstock) or paper tray source, or if the print job is from a computer or an internal configuration type of print. I've done all built-in cleaning processes, switched toners multiple times, turned off cartridge check and moved toners around or removed them, and I just installed a new HP OEM transfer belt purchased directly from HP. The lines start about 1-inch in from either side and are about 1/2-inch wide, sometimes rather solid, but usually faint long 1/2-inch wide lines, varying in density as they go down the page and vary as to where they go down the page. They sometimes end up on the front of pages too, but most noticeable/frequent to be on the back. The lines are not from any kind of rollers that I can see and I get in and really look around. The lines are not in the same spot where rollers are. I've replaced the fuser with a spare, just to be sure. There is also no excess toner anywhere and I have a toner vac if there was, and have cleaned them fairly thoroughly. There usually is excess toner in the same area on the transfer belt and cleaning this off around the entire belt does not resolve it as it shows right back up  This is on 2 different 4700 units, doing this same thing. Switching parts back and forth doesn't resolve it. The 4700 transfer belts do not have their own waste hopper as that function is built-in to each toner cartridge. These are older units but usually  print beautifully and quickly on the front side, but make the document useless for customers with the toner stripes on the back of most every page. I have many newer printers but just don't want to dump these if I can figure out what this issue is. Thanks,