Are both your cranks wobbly?
If they are, then the problem is probably the bottom bracket. Depending on the type of bottom bracket you have, it could be overhauled and adjusted, or just replaced with a $15 cartridge bottom bracket.
The problem is that you need a few specialized tools to do this job... And the bottom bracket is the single part on the bike that requires the most force to install properly. The cranks also need to be attached to the bottom bracket with a lot of force.