Any great cummins diesel guys on here?

70Duste, I have been presented with this problem in the past, and very recently. 1200 psi cranking from the CP3 is pretty low. I would check the fuel rail pressure relief valve. It is located on the fuel rail just behind where the intake is. It is about 2" tall and fastens directly to the rail. It has a hard fuel line on top secured with a banjo bolt and the line returns back to the fuel filter housing or fuel return system (i believe). Its role is to relieve fuel pressure should it get too high. So to say, fuel should not get by there if all is well. If it has failed, its like having a split hard line to an injector and the system cannot build proper fuel pressure. But.....it is an internal leak. To test, simply loosen the banjo bolt (3/4 or 19mm wrench or socket) on top a couple threads and watch while someone cranks. If you see fuel come out, any fuel, its done. Replace the valve. 5 minute test. This condition could certainly cause your P1222, long crank/intermittent no start. Let us know how it goes.
Chris