Spinning Horizon

Here's a weird one I'd never heard of before but maybe one of you guys can
explain. I installed a new attitude indicator last week and went out for a
test flight yesterday. It's just a plain vanilla R C Allen vacuum AI. Well,
it worked great on the flight - erected quickly, and nice and stable in
flight. But then when I shut down, and the gyro was spinning down, the
horizon started spinning, fairly rapidly - maybe one revolution per second.
Then of course it eventually stopped as the gyro spun down.

So, has anyone ever heard of this? is it indicative of a problem? It's a
factory overhauled unit that I can pull out and take back to the shop if
necessary, or I can learn to live with it if it's harmless.

Thanks, and happy holidays all,

Mike