Hey guys, for a while now I've been having this problem

I have a d16a6 with AEM fuel rail
I was running the stock fuel pressure regulator untill I got a new motor from hmo. I put in a 9 lb flywheel at the time
Had a heck of an issue with cels
Finally I found out that my fuel pressure was low
I put the AEM regulator that came with the rail, on and set it to spec.
The car runs fine and everything
It's only cold starts and idle
I have to let the pump prime in the II position for about 1-2 seconds and it'll start fine. But if I just turn the key real fast it really won't start till about four- five cranks
I put the stock regulator on and the symptoms go away but my o2 sensor cel comes on (this was not happening before the new block and 9lb flywheel).
I even have the regulator set to the lowest spec required for a d16a6

Another symptom I have is my idle and fuel level change slightly if my car is parked downhill, level, or uphill
I hear the pump prime and the car runs fine besides the cold start so I think it might be the ignition switch because my main relay is new and so is everything else in the car except the starter

edit: my car always had the hard cold start with the AEM regulator, but not the stock one (reason I was using the stock one) but lately I started getting the cel

Should I put the stock rail and regulator on?
Would the rail affect the psi?

Cliffs: Basically in my opinion it comes down to this;
-Fuel pump starts car fine with stock regulator, stock regulator is not putting out enough psi
-Fuel pump has hard cold starts with AEM regulator set to specs

Also, is changing the pump something I can do myself?
Any diy writeups on this would be greatly appreciated

edit again:

my fuel filter is 5 years old, will change asap