carbon build up is ok...so that cylinder is probably running pretty rich, but not so rich that its fouling the plug. you did check the gap with the plug installation, right? Probably not the problem, but ruling it out just in case...
I'm in a similar sitation with what i'm thinking is the EGR...in my case i have no codes, no hesitation whatsoever...and the engine runs great off idle...the car will also start fine, idle after the start perfectly fine...warm up normally...and hold idle if i just stay park and blip the throttle...
but...whenever i drive the car and come up to an intersection or something...revs fall like the ignition was turned off...and it catches, barely, at below 500 rpm...and bumps around for 5 to 10 seconds until it levels out at idle...I do have some afterrmarket cams that seem to have made the problem more pronounced...but i'm pretty certain its not just the cams...
From what i've read...a problem like what i have is directly related to a sticking EGR valve...some people have what i spoke of, some have it with a p0300 code...and some have a little hesitation...I haven't seen one where it was giving a specific cylinder misfire though...I too am at pretty high mileage (157,000), and have never once even touched the EGR...never needed to before...
as far as the injector swap, i'll dig up the threads about it when i get a chance...but if you're plug only had some marginal carbon build up...i think its now less likely that an injector is at fault...you have enough egr like symptoms; i'd say thats the best place to start...
this thread is huge, but goes over tons of people that have done the work...it started in 2005, but continued all the way to very recently...pretty good info if you have the time to read through a lot of it...
http://www.mazdas247.com/forum/show...GR-Removal-and-Cleaning-Rough-Idle-Fix/page76