Sounds similar to a problem I had last year, used to collect points at morrisons so always filled up there
on a long run had to fill up elsewhere bp I think, few days later I noticed it seemed alot smoother on accelerating
I thought it was maybe the long run had done it good, the next week problem came back, had to do the same long journey
Again, but this time made sure I had filled up, got to collect my points I thought, few days on car was still noticabley sounding/driving rough
up to 3-4 tho rpm then seemed to behave itself beyond there just as it always did, apart from the one time I filled up somewhere else, so I tried
my theory out filled up at the local total garage, bought the higher octane stuff for a treat, instantly car started running better, so next time I filled up
same place but got normal fuel, ran fine. I thought it was just a myth but changing petrol when I did saved me fortune as was going to start my problem
solving list the same as you have, I hope u manage to find the problem soon without too much more expense! Good luck.