Digital BW, The Print

After printing over 100 13 x 19 prints on the 4800 with the Harman 
Gloss, just yesterday I began to have a problem.  On small areas of a 
print (a couple of inches across, irregularly shaped), in darker areas, 
there was a pattern of very fine straight, parallel lines in the 
direction of print head travel.  Looking under a microscope it seemed 
that these lines were gloss differential, not desnity differences.  The 
lines cannot be seen if the print is held at a glare-free angle.  
Assuming they were from the printhead very lightly scratching the 
surface, I adjusted several parameters and increasing the platen gap 
to "wider" (the largest setting) seems to have stopped the problem.  It 
became dry here yesterday and I think the dark areas were being 
saturated and the paper was warping like air-dried gelatin papers did.  
So, I'm left with some questions.

1.  What is the significance of platen gap and paper thickness?  I 
assume that the distance of the print head from the paper surface is 
the platen gap minus the paper thickness.  I have measured the Harman 
paper at 0.31 mm and have used a paper setting of "3" along with 
the "wider" platen gap setting.
2.  Epson specifies doing the auto head alignment procedure with either 
EEM or Premium Lustre.  Is there a reason to not do this with a gloss 
paper?  With the paper that will ultimately be used for printing?
3. Is it not important to do the alignment procedure with the paper, 
platen gap and paper thickness settings as they will be used for 
printing?
4.  The paper configuration settings in the Epson driver allow vacume 
adjustment for standard and lower settings.  The printer itself allows 
a high setting while inserting a single sheet, but this is overridden 
by the driver when printing.  Is there a way to use high vacume for 
printing or would this prevent proper paper transport?  It seems to me 
that it might suck down the warps and allow a smaller platen gap.

Any thoughts much appreciated.

Walt