I've posted on this problem before, but this time the culprit was a bad servo.

In normal mode, the heli flew fine. But, in idle-up it jumped into the air about 4 feet up. Then it tried to slam itself into the ground. I saved it, but it was crazy. Jumping up and down by more than 6 feet! Took everything I knew to get it back on the ground. Managed to set it down without breaking the skids, but once it was there, the main blades had a strange sound. Apparently, one of the flybar links became disconnected from the swash. Reconnected and tried again. Flew ok in normal, but idle-up had twitchy results. Finally, one of the flybar links came loose in flight and I set her down to find the link was broke on one side and disconnected from the swash. (did it break first then disconnect, or disconnect then break??)

After last battery, I looked it over closely. The swash would jump when moving the front servo up or down by hand. Why? After a while, it was apparent the front servo had a stripped gear. Maybe one tooth missing. It was right at the level servo arm setting, so it only revealed itself in idleup.

Replaced with Hitec 55 and flew much better, but didn't have a battery to try idleup.

I'm sure the swashplate was jumping up or down on the shaft as the missing tooth on the servo let it. Something to check.