Wazz...
Next time something like this happens....you might try taking your hand off the collective and very firmly brace the collective with your knee so it (the collective) cannot budge...not even a smidgen. Donuts to Dog droppings the wild ride will end almost immediately. That being said....there is something not quite right about the Collective friction set up on that aircraft I bet. Get the Gingerbeers to put the manuals aside....and do what they do best (well second best) and work their way through the collective control system link by link...bolt by bolt...and look for something subtle.
In the early days of flying the Chinook....we had a similar problem caused by a weak magnetic brake on the Thrust Lever (Collective Lever in Single Rotors), and every now and then took a wild rodeo kind of ride you mention. Trying to hold the Thrust Lever with one's hand only made it worse. The only way to effectively stop the Lever from bouncing was to do as I described....use a leg to act as a physical lock and firmly hold the Lever against it until the Bounce went away...usually only one or two bounces at most.
It would be worth trying anyway.