trying to refine my print results

I wasn't sure if this was the right forum but deltas do have their unique print quality issues so here I am.
My printer details are below.
The picture is of a part in PLA, 250 um layers at 30 mm/sec, 400 um nozzle.
The walls and flats are superb but the larger columns are doing weird stuff.
I've tweaked the pressure advance (usually use 0.1; this picture is 0.2; 0.3 got worse again) and that helped but it's interesting how as soon as the tiny columns (1mm diameter) are finished the taller wider columns start over extruding.
Perhaps I am expecting too much detail from a large printer but I have been working on the accuracy and stiffness for quite a while, custom machined carriages and such.
But the tiny columns are doing better than the big ones!
It *seems* like the problem is over-extrusion caused by built-up pressure but I can't really make that work in my head.
Suggestions? Thanks.

The printer is a large one, 300mm print region, 80mm wide rod spacing, about a meter tall. Uses EZstruder and about 900mm of Bowden tube.
I recently swapped in spring-clamped ball-cap rod-ends from Trick Laser and I really like them.
My retraction settings were dialed in some time ago, but they are lift .7 at 30mm/s, retract 3mm.
Slicer is Slic3r 1.2.9.

If you can share the STL, I'll try it on my delta.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

With the stringing taken into consideration, I think your assessment of over extrusion is correct. Possibly a bit to hot as well. lower extrusion and see if it affects the lower square base top surface, try a slightly lower temperature, and If you can go up to 40mm/s retraction and make the travel moves as fast as your printer is comfortable with. Just another delta operator, only advice use at own risk, hope it helps!

Thanks, here is the file.
BTW, this is a cutout from a much bigger calibration plate, and that had better results although not as good as I wish. I tried to use the auto-cooling features in Slic3r that slow down prints below a certain layer time but it didn't seem to take. Also, I have three fans around the bed blowing toward the center but nothing focused closeup around the nozzle and it definitely needed more cool-down between layers.