Bob, I feel your pain, but, I found a solution that worked for me. I had the same issue printing on a Bayrata surface paper at 17x25. I set platen gap to "wider", paper thickness to 6 instead of the default 3, then I set the drying time to maximum (which will cause the print head to pause longer between passes) and also dialed down the print density to -15. You'll have to mess with the last two to find what works. I am assuming that due to the large surface and the fact that the 3880 doesn't use vacuum to hold down the paper that the paper surface swells with the larger surface ink load - particularly in the darker tones - compared to smaller sizes and therefore contacts the exit wheels leaving the marks. I do not get this issue when using an RC or lustre paper.
I really hope this works for you too as I've gone through the torture and expense myself.