After about 15 years of playing my rhythm was very weak. I could hit the beat but it just sounded very forced. My lead lines were fast and I could pick very quickly, but not very accurately. Forget strummed acoustic.
After playing that long, I had learned habits that were very bad. For example: my alternate picking was not strick. I would alternate pick all the notes on the same string, but always start with a downstroke on the next string. BAD. I found out that not keeping an up and down motion either at my wrist or at my elbow was holding me back from playing fast and accurate.
So, I started over. I took a metronome and set it slow. I would play an octave of a scale, or E-123, A-123, D-123, G-123, B-123, E-123 (this I suppose is quasi-chromatic) than work back moving up a 1/2 step: E-432, B-432, G-432, D-432, A-432, E-432 and so forth. During this I FORCED myself to look at my picking hand and say outloud "Down Up Down Up". It was horrible. I felt like I had flipped the guitar over and was now playing left-handed. My mind said "Forget this, you have played too long for this humiliation". But I kept it up. Once I could play all notes smooth and clean I bumped the metronome up 8BPM. I did this with various patterns, scales, etc.
Low and behold after a few weeks, my scales sounded a ton better, my improv skills improved and guess what - my rhythm was even solid and smooth. I had focused too many years and shredding with the left-hand being more important than the right.
Now my guitars do not have to have the action so low that a piece of paper barely slides between a fret and the string and the frets buzz because I can play more accurate and not rely on brute strength of the left hand to sound the notes.
Sorry about the rant.... :D