it sounds like you have a leak or another circuit being closed/open to turn off the circle welder. i have seen a secondary circuit running through the weld current before, i.e. when the circuit was closed a 110 0r 220 0r 460 0r 575 v circuit was superimposed over the low voltage/high amperage circuit causing all sorts of issues, caused by board issues all the way to simple circuit contamination from the welder being filled with grinding dust
does the 600 unit weld well with out the circle welder i.e. pulling a piece of parent metal instead of having it automatically being moved.
this is interesting (i am sure on your end frustrating) did the 600 unit develop this problem spontaneously or has it always presented this problem.
we had some issues with a power wave introducing a secondary circuit and have chased ghosts in some older machines.
there is something to be said about the bullet machines, even though they cost more to run you would almost never have any of these issues.
time to pull out the fluke and do some rudimentary testing.