I actually give bass lessons for fun to my friends who are in bands :) But if you're having trouble with the rhythm, try using JUST your plucking hand, and don't fret anything with the other hand. That way you can focus just on plucking the strings correctly. Once your fingers can pluck the correct rhythm easily, then you can add the fretting hands. Another excersice that may be very helpful for you, is just simply doing the chromatic scale up and down the fretboard. You can't overlook the fundamentals :) As far as that goes though, try to pay alot more attention, and give special care to the first 4 or 5 frets on the neck, as opposed to anything higher, because obviously the lower frets on a bass are spaced alot further apart. But if you can master the chromatic scale between the first and fourth fret, everything else should be a relative breeze :)
