1G 1G GSX shuts off after 15mins driving, struggles to restart, then ok?

[AWDPetmitsu]

1991 GSX, starts fine when cold and after 15 mins of driving, it shuts off by itself. I'll be driving and then it'll act like it's misfiring and bucks. After that, suddenly the tach drops to 0, but no dash lights come on and there's no power; the engine is essentially dead. I'll floor the throttle and sometimes the engine comes back to life and it'll repeat this cycle. If it stays off long enough, then it's stalled and the dash lights come on. I can restart it but it struggles to restart and stay running. After a few restarts, it drives ok for the rest of the trip which is about 15 mins more. I haven't tried driving it longer than that.

The car is all stock, no mods. It has idle surge but runs fine. I've just replaced spark plugs, wires, power transistor, ignition coil, ISC. The ECU was replaced 6 years ago. The CAS is original. I'm thinking I have a defective CAS. Anyone have any other suggestions?