thrumming in harness MIG / GMAW

Hello all
Upgraded torch / gun from some "Crapomatic" to Tweco #4 Classic.
Tweco doesn vastly better than the "Crapomatic", but still sometimes get this very characteristic problem - the torch and harness vibrate, and the wire "judders" coming out of the torch. What are the likely causes?
What do you do to avoid this happening?
Rich Smith
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Wire 1.2mm, which is .045".
The nozzle is quite small - something like 5/8inch. The copper has gone a dark chestnut colour. Not perceiving nozzle as being a / the problem - but could be mistaken.
BTW - like the Tweco nozzle mounting, where can slide the nozzle up to get contactor tip level / protruding for dip-transfer, then slide it out to recess contactor tip for spray.
Gas is Ar-20%CO2-2%O2. Suspect that need more voltage than you'd otherwise use, to get a spray-ish transfer.
Currently running most of the time at around 290A to 300A.
Anything else you'd find useful to know?
Thanks in advance, Rich
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