Having used a Model 66 Powermatic for 25+ years, many times with a full 3/4" stack dado on the saw, I can't imagine the saw is at fault, unless there is some specific fault with the motor.
Granted, these new dado stacks with their anti-kickback fingers and four teeth per chipper instead of the customary two are heavier, and cause a greater load on the arbor, but it's a balanced load, and bringing it up to speed shouldn't be an issue. Consider the torque involved in ripping through two inches of solid oak, for example. So I would also rule out the dado set. I've seen many saws turn the same set, some running with only 1 1/2 hp and 110v extension cords. (Though I don't recommend this unless you're very patient.)
The three phase connection is worth some investigation, in my mind, as the likely culprit. I used to have a 5hp 3ph motor on my saw, and at the time had a second saw hooked up to the same power feed in the shop. We did what your electrician told you was possible, took a 230 (208) leg off the 3ph power. It's been many years, but it seems to me that the electrician who helped us hook this up at the time warned me that it had to connected in a specific manner, using particular leads, because of the nature of the 3 phase. This has something to do with the lack of a neutral, I believe, but I'm not an electrician. You might consult the following website for some further information regarding the specifics of this type of connection, and reference the following info: http://www.3phasepower.org/
1 Phase Loads on 3 Phase Power
Single-phase loads may be connected to a 3 phase system in two ways. This can be done either by connecting across two live conductors (a phase-to-phase connection), or by connecting between a phase conductor and the system neutral, which is either connected to the center of the Y (star) secondary winding of the supply transformer, or is connected to the center one winding of a delta transformer (High leg Delta system). Single-phase loads should be distributed evenly between the phases of the 3 phase system for efficient use of the supply conductors and supply transformer. Click here to read more about connecting a 1 phase load to a 3 phase power supply line.
If that's more than you wish to digest, I heartily recommend you consult an electrician accustomed to commercial industrial wiring. He ought to be able to take a quick look at the breaker box and the connection to the saw and sort you out.
The only other thing I can think you might be able to check on your own is the wiring configuration of the magnetic switch itself. It is turning off, you say. They must be rewired internally to conform with the particular power supply provided. There should be a wiring diagram inside the switch box or in your documentation that shows how to do this. I can remember changing the leads, and at one point changing the actual thermal breakers, when I moved from one shop to another over the years. That may have been when I had to change the 3ph motor for a 1ph one. If the wiring to the saw is correct, this might be the direction to look.
Short of loose drive belts, I can't think of anything else to offer you as a troubleshooting guide.
Let us know how this works out, and let me know if there's anything more I can do to help!