Mortiser Adjustments
It takes time to learn the tips and tricks of the tools we use and often if everything is operating correctly there is little need to explore manuals. When I acquired the Powermatic Mortiser last year, I spent some time cleaning it and browsing through the manual. The machine has performed well, until I noticed that it was not holding the longitudinal (best description I could come up with) setting and creeping as I made longer mortises.
My first instinct was to check the alignment of the drill bit, but it appeared to be correct. Next I compared the movement of both the lateral and longitudinal adjustment wheels and noticed a distinct difference. The longitudinal adjustment turned freely while the lateral was stiff.
It was clear that I needed to find a way of holding the adjustment and looked for a knob on the handle similar to those on table saws…not there. After a little thought, I discovered the gibs on the side of the table and then the wing nut.
Additionally I went back to the manual to see if there was any advice on how tight to set the gibs. Interestingly there is no mention of these adjustments and the although they are pointed out in the parts diagram and list, the purpose is not obvious.
I started writing this blog to share a little bit of information, however it has really made me come to realize that many of the tools and skills we acquire over time require a certain amount of basic knowledge of mechanics, a willingness to try and fail and access to information. Fortunately the internet can provide a great resource and if you are in luck there may be someone you follow or a neighbor that is willing to answer your questions.
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orepass 
Gibs are to take up any slack or play in a sliding mechanism. They should be adjusted until there is no play but loose enough that the table can still slide. Since they push the table over into the other dovetailed way, they should not harm alignment. They can be adjusted too tight to cause premature wear to the ways. Parafin wax should be used to lubricate the ways so not to attract dust and dirt.
I have the same mortiser as you do. When setting it up, I did something wrong with the gibs during set up (first time I ever had a tool with gibs) and ended up calling Powermatic. I can’t recall what they told me but they were very helpful and it sounded like what I was asking was a reasonably common customer call.