im about to do a repair myself on a 3.0 v8 motor... same issue
it started with a slightly lumpy idle, the car now does not idle properly at all untill it get to temp.
the problem? other than the regular valve cover gaskets.
there another set of gaskets which keep the cam oil spray from seeping through and onto the plugs and coils... this gasket leaks, fouling and half fouling the spark. the spark inturn jumps all over the show...
my M60 idled butter smooth before this....