Just to check a few other basic things...what psi do you have it at? Did you make sure that any gas valves are open, including the main valve on the CO2 tank? Is the regulator connected tight and properly? Is it pouring only more slowly, or more slowly and foamy? When you hooked the liquid line up to the cleaner keg, were you using the same CO2 set up? If so, it would appear to be something particular to that keg.
Typically, a slowing pour regardless of which out line would indicate the CO2 pressure pushing it out is the issue. When the keg sits for a while, does it pour a little better but then slow again? This would indicate no CO2 getting in to push, but the CO2 is coming out of solution to equalize the pressure in the keg.
Did you remove both stems and poppets (gas and liquid)? How about the dip tubes? I'm assuming you don't hear the CO2 running or air coming out of the keg that would signify a leak.
Last question...was the beer so good that you just drank it all and haven't come to terms with that?
Hopefully those questions make sense and trigger something!!