a couple of questions come to mind about this issue...
- does it do this while the carriage or mast is moving or when you release the lever/handle to stop movement, (when movement is stopped the mast/carriage bounces) ???

- have you checked the mast and/or carriage to see if there is anything restricting movement, does it do this in one particular spot or at any position? are there any abnormal noises when this happens?

although it does sound like an accumulator issue i would rule out any other possible problem before replacing any parts.

generally when you get a bounce when movement is stopped (if thats what this is doing) it is due to air in the line(s) or in the accumulator.
If there is a bleeder screw on the hydraulic side try and bleed out the air, if no bleeder just crack the line with the forks slightly elevated and let it bleed out for a sec, if there is air it should come out then.
IF you do get air i would look for where it is getting the air from.

If i can think of anything else i'll come back and post it, but to me it just sounds like air in the system.