Runs smooth then rough then smooth

[87gmc]

I have a 2000 Excursion 7.3.

When i start the engine runs fine then after a couple of min starts running rough kinda of like the wiring harness is faulty like running on 4 cylinder. After a few mins goes back to normal.

This only happens with its cold kinda of like a set pattern. No SES light.

The ECM was replaced about 8 months ago and CPS sensor.

Any ideas?