Heh, interesting question.
As for talking - I'd say my pronounciation is just bad, no matter which language I speak. People tend to ask me to repeat stuff a lot
For grammar and stuff, I've always had a rather good feeling - never bothered to remember the comma rules, but I usually guess right, for example ^^
And in writing, I guess it really depends on the topic. There are just some areas, mostly academic, about which I've only heard/read in English, so trying to write about them in German feels really awkward. (And in others, like topics I learned in school but don't usually encounter, I always hit the missing-vocabulary-wall in English.)
I do the language-mixing almost only in my head - it's just more effective - but yeah, it has happened that I really couldn't figure out a good way to say something precicely in German and used the English word :x