Unsolvable Woofer Pumping (Phono only)

@three_easy_payments Nope, it is pretty consistent throughout.

Well you've certainly taken all the right steps to narrow it down, so good on you. With a lot of the most likely culprits eliminated we are down to only a very few good options left. One being, its perfectly normal flutter, you're just not used to seeing it. Most people with entry level tables have been raised on CD and don't know some amount of this is normal. Not being there we can't know if this is it or not.

Moving on.... your table has a felt mat and let me guess, you're not using a record clamp. So records lay kinda sorta flat. But not really. Problem being, anything other than absolutely flat gets amplified by the time it gets to your speakers by about a hundred thousand times. 

The solution is a record clamp. Not weight, clamp. Specifically, you want one that's cupped on the bottom, or has something like an O-ring going around the outside. Anything to create a space between the record label and the center of the clamp. Then you also want a thin washer, just barely thicker than your mat, that fits over the spindle. The idea being the clamp will press the record down forcing the edges into the platter making the record lay absolutely flat.

A good clamp used like this will almost always be a nice improvement, which since it will also work on future better tables makes it a fine investment no matter what. 

Since you say its noteworthy mostly when just treble this implies to me you don't really notice it with bass-heavy music, which in turn implies to me that whatever flutter you're seeing is no more than normal music level or less. Which to me means perfectly normal, you're just not used to seeing it. But whatever. Try a clamp and see.

Record warp was my first reaction as well.  Is the woofer pumping worst when the tone arm is near the outer edge of the record and best as it moves close to the dead wax?

@mike_in_nc Thanks, I tried several different records. Do you think a record clamp would help? I've been meaning to get one.

Record warp is another possibility.