I am thinking you need to verify your checkvalve for the HPOP reservior is not faulty since it comes to life after it is warm. This can be done by checking the HPO reservior VIA the allen head plug on top after it has sat for at least overnite. Should be within an inch of the top at worst.

Filter change is always a good idea, at the same time you check the HPOP level, just pull the bowl cover and check the fuel level there also. In addition, there are restrictors in the fuel lines where they enter the heads. Complete with screens. Mine are in the shed. Maybe they somehow get over come if clogged once the fuel pressure builds. That last is a swag but it is under troubleshooting for a lot of symptoms in the manual.

Disabling the EBV is also a good quick check but it should not cause it to chug when idling/warming up. If it is malfunctioning, it would simply fail to disengage when you get underway and then present a problem, but still worth a look.

I somehow suspect Alan has looked into this with his vehicle with similar problems but maybe not. Just my 02.

Perhaps a bad idle validation switch or throttle pedal assembly? And or both. Anyone else?

Also, a clear return line is not necessary as any type of cheap see through filter plumbed in there or similar item will serve the purpose.

BTW, I run 15W 40 year round. If it's clean, it's not your problem.