Like many people, I have my "serving pressure" at 12 psi at 40 degrees. I would suggest that having two (or more) different pressures- one as a "carbing" pressure and one as a "serving" pressure is the problem.
"Force carbing" means not using priming sugar and naturally carbing a keg. It doesn't mean turning up the gas and shaking, or turning up the gas and turning it down- that is usually called "burst carbing".
So I think you mean you "burst carbed" the beer, and then set it to a lower pressure ("serving pressure") and as zach said, that doesn't work very well because the beer is overcarbed and will come out of solution as foam.
What did you carbonate at, and what is the keg at now (psi-wise), and what are the temperatures of the kegs? If you still have it at 6 psi, the beer will start to lose carbonation.