Sometimes when i leave it a day or two, i can play better than i did when i was playing day in day out, sometime not.
If i can't play too well i'll go back to some simple chords or a boogie riff and warm up for half an hour. it seems to be mostly attitude or inclination, sometimes when you're not thinking about a song it flows perfectly then you start thinking " Wow, i aint so bad today" ...and then you ARE so bad today!.
If i totally suck i play my acoustic for a while then my electrics are comparatively easy to play afterwards.
Try not to be tooo hard on yourself and like you say, maybe warm up with some excercises/scales.
Cheers.
Accuracy,you say? hmm interesting concept..