If it's the playing, not the instrument itself, that's inconsistent you'll get the best results tracking to tape. But if you don't have a Studer kickin around, get your hands on a Distressor.
If the problem is the way the bass is set up, open up your favourite EQ. Each bass note will have a fundamental frequency. Find the notes that are sticking out and notch 'em down a few dB until it's all nice and smooth.
Here's a handy chart to have bookmarked:
http://www.phy.mtu.edu/~suits/notefreqs.html