Are you sure all of the movement is between the quick release plate and the camera? From your description of what is happening in your question it sounds like the 494RC2 head is settling a little when you let go of the camera with your hands.

The Manfrotto 494RC2 is rated to support up to 4kg/8.8lbs. That assumes the load is relatively centered over the head. The E-620 + Zuiko 70-300mm lens only weighs in at about 1150g/2.5lbs. If you are attaching the camera/lens to the tripod using the camera's base plate it should be able to support the weight, but you need to be sure the ball head controls are adjusted properly. This will be especially true when the lens is set to 300mm and the barrel of the lens is fully extended. With some telephoto lenses a tripod collar that attaches the lens directly to the tripod head can be used to center the weight of the camera/lens over the tripod, but the design of your 70-300mm lens does not appear to allow for a collar.

Try tightening up the smaller knob marked friction and see if that helps any. You should make adjustments with the larger main lever released, then lock it to test how much "give' you have dialed in. Some users actually want to be able to shift the camera/lens with varying amounts of pressure when the main lever is locked and the friction knob allows them to adjust for the weight of the camera/lens. If you are shooting "hands free" via the timer or a remote, you want to set the friction control much tighter than if you are holding the camera to aim and focus while shooting.

Another possible source of instability is play between the quick release plate and the quick release base on the top of the ball head. I've found that the 200PL-14 plate can eventually wear under heavy use to the point it gets a little loose and needs to be replaced. The looseness usually manifests itself in side-to-side rocking rather than up/down, but it will depend on which way you connect the plate to your camera. I usually connect the plate to a lens collar with the long side of the plate running parallel to the optical axis. The newer 200PL is now listed as the replacement and fits all versions of Manfrotto equipment with an RC2 quick release connector.