mustang, are you are sure that the cutter disk rotates smoothly on its bearings even when the vibration is happening? The nearest you can come to checking this is to run the mower until it starts vibrating, stop the engine (including pulling off the spark plug leads) and declutch the cutter, then rotate the cutter disk by hand, looking for signs of roughness. This is not an infallible test - for one thing, if the belt tension is still on the cutter pulleys, the cutter shaft bearings will be pulled to one side and that could keep you from noticing any slack there may be.
If the idlers are slack, either between pulleys and bearing adapters or between bearing adapters and shafts, they have to be repaired - whether or not they are causing the current problem, they will cause one before long. Posting some pictures would help - at present I don't have a clear picture of the design of those idlers.
My suggested course of action is to repair the idlers, then see if the vibration is fixed. If it is not, it will be necessary to take a close look at both the drive (i.e. engine) pulley, and the driven (i.e. cutter ) pulley.
