How can I replace an I-T-E panelboard?
This sounds like a bad connection upstream of the panel. No load, you get voltage - apply a load, voltage goes away.

Try turning off all the breakers on the affected bus, with it connected. Does voltage return?

Try (since you seem comfortable handling the feed wires - right, you do say sub-panel) marking the feeds and see if the problem follows the wire, or stays with the bus. If it follows the wire, the problem is not with the panel.