Invision 456Ps weld variation

[roboweldr]

We have 3 Invision 456Ps set up to weld small parts (in a factory). Two of them have 70 series feeders and one has a 60 series feeder hooked up. They are all set up on the same kind of workbench.
Operator is complaining that the welder he uses daily welds erratically and will not weld the same as the other two units at the same settings. Ground is good and has been moved from one part of the bench to another to try to eliminate the problem. Didn't help. Operator also stated that this power supply has been swapped out several times.
Electricians looked at it today to check power coming to the welder. Readings looked good.
I looked in the owner's manual for troubleshooting help - proper size and type of cable and clean and tighten all weld connections. I believe these to be good on this unit but will check.
Just wondering if any of you have other suggestions to try.
Serial number for this unit is MB460005A.

[Frank865]

Swap operators for a day & see if the problem moves.

[cruizer]

Nothing to do with the machines, the s-60 is a better feeder than the S-70. so yes, has to be something to do with the operator, would be my guess as well

[roboweldr]

The op, whose nickname is Rambo, won't appreciate your responses.
He's actually the best welder back there. And, he's not the only one telling me that this welder/ station acts different than the other two.
I thought about swapping one of the ones they say welds good for this one to see if the problem follows the power supply or if it's something at that work station.

[cruizer]

Problem is that once any operator says a certain station is bad, then that station is bad forever. Wouldn't matter if machines were changed or not.

What I do for stupid failures, is take the machine apart while who ever is there, find a fake problem, show it to the operator, change a fake part, and reassemble. Suuddenly the probem is gone.