Hi Walker,
Epson techie pointed me in the right direction: paper feed adjustment problem.
I found the same issue and the recommended values in an earlier post:
Basically the recommended “photo paper 170” custom setting automatically sets the feed adjustment value higher, resulting in banding.
Playing around with adjustment values works.
On a different note, the glossy 170 setting is still thicker than the Pictorico OHP, so when you run a thickness pattern even the lowest value (1) is slightly off. I ran a plain paper setting thickness test and indeed there was some leeway: the correct thickness was around 3-4. Which means that technically the printer is set to a slightly thicker media when printing on OHP (which is 5.7 mil thick, as I understand). Maybe the difference is negligible but it might also affect banding.
Cheers,
Csaba