Yes if you leave your PSI at 20 you'll get lots of foam. Likely why you're turned down to 5 (slower pour = less foam) and you are likely over carbed by now. A more typical serving pressure is 10-12 (like Chris said around 2.5 vols).
A lot of people here carb at 30psi for 2 days, then serve at 10-12 - works pretty good for me and tends to give me around 2.5 according to the charts I've looked at. I like this chart:
http://www.kegerators.com/carbonation-table.phpAs it is, I'd leave it at 5 and serve the rest that way if its at a level you like (if you turn it to 10 it will pour faster and make more foam) - it will take an extremely long time for any carbonation to come out of the beer if you leave it there, and should pour consistently from now on.
Theres a lot of stuff out there on the topic - formulas for beer line length, pressure for each etc... I like the 30psi for 2 days rule though. If you're not running line to other rooms, etc... then it seems to work pretty good.