Spend a lot of time playing with a metronome.
I don't see anything different that could be done in order to practice timing.
A drum machine is not a metronome.
What I like about the metronome is it's simplicity, it won't introduce rhythm, just constant "claps".
The problem in practising with a drum machine is that the beat configured in the drum machine can be quite complex, and mislead the musician in terms of time.
Imagine this drum machine configuration (with HH playing quarter notes):
Notice how the snare beat marked as a
ois clearly "out of time", and is, in fact, one eighth-note, but can easily confused with a quarter note.
If you practice with a metronome you don't have to worry about anything, you know that each "clap" is the note you programmed it to be.
I still think that a metronome would help.
Some times, when I'm studying some complex rhythmic parts in one piece, I usually put the metronome to "clap" at eighth or even sixteenth notes, to be able to understand it.