I got Westfield's NB1 alternator control on my DIYPNP and it worked fantastically for a while. I drove my car yesterday, and I noticed that as it got hot (heat soak, CLT was fine), the car would start to misfire. I noticed that my voltage was higher than normal, but didn't think much of it.
Later in the day I took the car out again and it became undriveable. I took a better look at things. My MS looked fine, including the Westfield board. However my alternator cable was very loose on the alternator, and my Crank sensor's wire shielding was shot through.
I tightened up the alternator, and tried the car again. It started fine-ish, but as I drove my voltage would slowly raise from 14v. I noticed however that if I let the car go into CL idle, the voltage would stay at where the westfield board was set to. As soon as it got out of CL idle, the voltage would rise.

Bad alternator?

I checked the CAS wiring a bit closer, and the ground wire had only 1 strand left. I cut out the damaged portions and replaced the broken wires. No chance to try the car yet.

When the car would misfire and become undriveable, the tach would cut out and I'd hear a relay clicking under the dash.

What exactly is cutting out?

 

I would guess your fuel pump is cutting out. MS controls it and if there are no RPMs FP is cut off.