A million times over: buy a metronome. Get used to playing parts SLOW with the metronome with no other accompaniment. If you can't do this, you can't play em regardless of how well you think you do with the music accompanying you. Speed covers up lots of deficiencies, as do other instruments.
If you really want to improve your timing, start slow and worry about accuracy. If you make mistakes at the tempo you are at, go slower. When you are comfortable slow speed up a little. Rinse and repeat.
It's REALLY hard to play slow with a metronome at first. The advanced training is to go even slower with the metronome. Once you practice it and get good at it, you will be amazed how much your timing improves all around.
Experiment with playing with the metronome only clicking the first beat of a measure, or only clicking 2 and 4.
Nobody in the universe has perfect timing. Timing is something you practice forever and hopefully continually improve on.
Edit: any practice you do with scales, rhythms, etc. should be done with a metronome.