I've taken the breathers off the rocker covers of my 4.0 V8 and they are completely full of milky crud, i've cleaned them out but i'm guessing the crank breather will be the same and maybe even inside the rockers covers.
Would this cause a misfire if the engine's not 'breathing' properly? because i've got a very annoying misfire under load which i'm desperate to cure.

Blocked crank breathers will usually end up either blowing oil all over the place or worse still blow a hole in either the sump or the crank case....

They have been known to case a knocking noise as the constanting hammering of the pistons against a pressure build up, but not a misfire in my opinion....

Misfire usually occurs due to mis timed fuel ignition, under load would indicate a HGF, duff plug(s) Coil pack, leads, MAF etc....

Yes ok thanks, i'm just randomly stabbing around in the darkness.