I'd remove the EGR valve proper and hand clean it ~ so often they get gunked or carboned up and cause weird running problems .

If there's a bunch of crud in the fuel tank you may have allowed it to run better until the new filter began to get blocked .

I know removing the tank isn't fun but sometimes the fuel intake screen gets slowly restricted.....

Sounds like you're close to getting it done, don't be in any hurry to increase the engine displacement, gasoline isn't going to drop in price any time soon .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better