I'm with others betting it's not a stem issue. Poor fit has usually showed up for me in sore shoulders, back, and neck. But my take on finger numbness is this:
1. It was worse when I started riding. I think mainly because I was doing my best to just hang on and I gripped the living H**L out of my bars. I think I was making up for weak legs by straining my upper body.

2. Lose the jewelry if you wear any. Had to quit wearing my wedding ring for some time because my fingers would swell and it would get worse.

3. Look at hood angle. Make sure your hands aren't cocked back.

Even after all that if you work in a profession that is prone to carpal tunnel you may want to look at excercises and stretches.