I was also getting an unstable arc much of the time. Good ground,
wire wheeled the work piece.

You should grind it till its shiny.

Wire feed seems to be pretty consistant.
When pressing the trigger, could clearly hear the gas at the nozzle
end. I could get it to stablize a bit by welding closer to the
workpiece, but ran the risk of welding the wire to the tip (which I
did at least once)

There are a lot of things that cause unstable arc in a mig.
Insufficent shielding gas, too low of heat setting, to fast wire
speed, to fast travel, dirty work piece, or a worn contact tip.

If your using solid wire it has a nickname(hardwire). It is very hard
as deposited. It is difficult to drill and usually has to be ground.
It doesn't like contaminated metal paint , rust ,dirt ect. If your
welding rusty dirty stuff the inner shield works much better and if
you get really tricky you can use gas with the inner shield and
acheive good results. 75-25 is usually used for welding light gauge
metals because it isn't as hot and the heating zone is not as wide.
Straight CO2 is used when welding heavier materials its cheaper and
you get good penetration.

Charlie