EGT's too cold and too hot - help needed

[Berkut]

That could be. The high EGT on #3 probably means that only one plug is firing, maybe a plug or a lead.

 

[Gorilla]

I'm not a mechanic, but I do know about thermocouples (EGT probes).

Thermocouple signals (probably a type K) are incredibly weak, just a few millivolts. The lines should be shielded, but there is a distinct possibility of interference from spark or radio systems. Did the routing of the lines change?

200 degrees sounds like a difference large enough to me that the EGT measuring system is suspect rather than the EGT on the cylinder itself. Possibilities...

Bad probe - you've addressed that with a good plan.
Interference - If the shielding of one of the lines is bad, swapping probes, again, should identify this, but be sure the lines are not routed too close to other lines which carry RF or electrical energy.
Bad black box - If one of the four input sections to your EGT display has a bad interface, you can swap probes all day and have one cylinder always high. If probe swaps don't work, perhaps you can test just the "bad" line with another thermocouple meter, i.e. unplug the bad line from your display, and adapt/plug it into another type K thermocouple meter. These can be bought off eBay or direct for anywhere from $50-$250, or maybe you can borrow one. If you do this, and suddenly the EGT on that cylinder is normal, then your display is bad.

Good luck!