thermal dynamics ac contactors

[canadian custom]

I've got an older (2000) thermadyne syncrowave 250 that my guys use for tacking alumn. the arc seems to stop and start randomly.
Could this be the AC spark assy?
The HF foot pedal seems to work fine on Dc.
But Do to a large contract I NEED this machine to be able to weld on Ac NOW!
Can anyone help?

[ASKANDY]

Cc

Could be alot of things... Are you using a water circulator or tap? If it's a tap water system, you are most likely losing HF through the water system to ground. You could try turning up the HF intensity (if equipped) or spread the contact points open to .012-.014. This will give hotter HF. The points could be pitted but on a 2000 unit, I doubt it unless you are in a high humidity area. What tungsten type and size? What material? Anodized? Are you prepping the tungsten on a dedicated grinder?
Could be the HF starting capacitor. They sometimes go bad.

A-