[differ]

I am having the same issue with my 2002 taurus; 110K miles
Since Nov last year it has had continued misfire issues:
Misfire on cyl 1; replaced all plugs and coil pack, and swapped injectors 1 & 3; cured the problem for about 3 months, then it recurred on cyl 3. replaced the #3 injector, problem not fixed. New plug leads helped a little, but the persistent P0303, P0316 codes remain. Cleaned and checked the EGR valve which seems to work ok. Finally I gave in and took it to Ford dealership. Their diagnostics pointed to intake manifold gaskets - compression ok and they did not fault the EGR valve. They quoted >$650 parts and labor to replace the gaskets and clean the throttle body and wouldn't guarantee the misfire would be cured. I did the job myself for $50 in parts and 2 hours of my time; the gaskets were deteriorated and brittle. It's better, but it's still not fixed - the check engine light went away but the codes are still pending, and I can still feel the misfire.
There does appear to be a significant amount of oil in the intake manifold, the car is using more oil than it ought to and #3 is the closest cylinder to the PCV breather, so the next thing is the PCV valve and I'll replace the rear O2 sensor for good measure (I changed the front one last year).
I'm beginning to think that this is a coincidental wear-out of various components. I've had 8 years of trouble free motoring from this car and I'd hate to have to junk it for this rough idle issue. Any suggestions?