Currently, I have two Pittsburgh Modular Structure EP-208 cases but when maxed-out with modules...
-- morphon
Given that this implies that BOTH cabs do this, my immediate reaction is that you're overtaxing the power supplies. But then, when I look at Pitt's site to check the current capacity, I can see that you've got 4A on the +12 and 3A on the -12. So, unless this thing's stacked with tube modules, you'd be hard-pressed to exceed that in 208 hp. The next thing to check, then, will be for flaky module behavior; it's a dirty little secret in modular that all modules are NOT quiet, well-behaved devices, and by unplugging/replugging each module one at a time from the busboards, you'll probably find a few "bad actors". The solution there is a little more...tricky, since the Pitt cabs also appear to have filtered busboards. This makes RF radiation more of a suspect issue here.
Now, the key to fixing stray RF is to isolate the problem component somehow. In a modular synth, this is going to be a major PITA, since there's nothing exactly "isolated" when it's in operation. It IS possible to add a ferrite across a ribbon (or two...one at the busboard end, the other at the module end) to keep this damped down...and if you're lucky, that might be all that's needed here. If NOT...OK, now it gets bonkers.
Also, check the power rails with an oscilloscope. No fooling. There could be power supply problems that, while you'll have the proper voltages on the rails, induce crud on the DC rails that can conceivably get past the busboard filtering. You want ruler-flat DC; if you see ripple or noise to an excessive degree, then the problem is the power supplies...and yeah, since they're OEM-type supplies, it IS conceivable that they could have the same circuit fault.
Lastly, and while I would be amazed if this was the case, how close are you to your current limits in each cab? It's worth noting that when you turn the system on, inrush current loads can be considerably higher for the fraction of a second when they happen than at any other time when the system is turned on. Best rule of thumb with current loading is to have at least 1/4th (and 1/3rd is better!) of your current max load "unused"...because the figures WE see in the MG specs are invariably for operational current, and NOT inrush values, and that headroom is needed for inrush. If you've exceeded 4 Amps in one of these...well, first of all, I'd be amazed...but if you're actually above 3A on your +12V rail, you're in that headroom area. Plus...the easier a load the P/S has to pull, the cooler it runs, and since heat is the enemy of electronic components, this helps lengthen the lifespan of the P/S itself.
Or in the end, you could just do what I do, and power your modulars with linear supplies. These are reliable, solid, noiseless as a rule, and while they're heavy and clunky, they ARE the sine qua non for powering modular synthesizers. I should note that I also do this with my amateur radio gear, and the results are worth it. Switching supplies are cheap, light, and simple, but even with decades of development, they're still just as capable of flaky behavior as they've always been.