Issues with extruding and skipping

Am at my wits end with this. I'm getting bad skipping on my extruder when trying to push filament.
My printer is an A8 with the following mods:
AM8 frame
E3D V6 Bowden conversion
Marlin 1.1.8

So the issue is I can push filament by hand fine, and it extrudes filament OK. However as soon as I put the extruder in line and mechanically extrude it skips repeatedly causing massive underextrusion. This is while it's away from the bed too, so it's not a backpressure from the bed issue.

Here's what I've tried so far:
E, X and Z1 motors, cables and sockets on the board. (to rule out motors, cables or drivers)
3 heatbreaks (stainless, stainless with PTFE liner, and titanium) - I have also tried seasoning the stainless one.
3 nozzles
3 extruder bodies
3 extruder gears (MK7, MK8 hobbed and MK8 toothed)
New teflon tubing
2 heatblocks (old and new style V6)
2 heaters
2 thermistors (value is set correctly in Marlin)
PLA and PETG at temps between 190 and 240 degrees

If I have the tension too loose in the extruder it just shaves the filament and if I tighten it to the point it properly grips it starts skipping immediately.

I'm not sure if there's much else left to try! I'm pulling my hair out. Any suggestions gladly welcomed.

What did you set as your filament diameter in your slicer?

If you set that to 3mm and your actually using 1.75, It will skip lots.

Did you adjust the extruder motor current on the stepstick?

Does it bind anywhere, gears aligned, hobbed bolt aligned with filament? Are you using a fan to cool the e3d heatbreak? Is the hot end temperature stable when extruding? Did you do pid auto tune?

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Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Check that the motor and stepstick aren't overheating.

Some more questions (apologies if these are a bit basic):

It *is* skipping, right? The extruder motor making a clack-clack-clack sound, not a buzz or hum.

Does it start skipping immediately, or does it work OK for a while before skipping? Anything time-related is probably originally heat-related.

Can you remove the hotend so it just extrudes into air, to check if the problem is in the extruder or in the hotend?

Is your filament oversize? Some filaments may vary in diameter by quite a few percent, even on the same spool.