Having successfully resurrected a Kieninger Obergfell Kundo clock, I'm wondering what the term fluttering means. Mine still runs a wee bit fast, and I have a handle on how to adjust it, but what's this mean?
It's when the escape wheel tooth doesn't falls without sufficiently locking on the pallet. If there is enough power on the escape wheel, then it will quickly slide off the pallet and the escape wheel may turn numerous times until it does lock...or not. So fluttering is a result of poor locking.
So check the depth of locking and if there be a need, adjust it. There are as I tried to point out, many factors.
Check the locking depth, then make sure everything is straight up, then maybe readjust the fork up or down if need be.
The main thing however is to be wary of other adjustments others may have made, before you change anything. This is possibly what I meant about straight up.
You can force most 400 day clocks to flutter if you move the minute hand when the pendulum is at about center swing. It sounds a bit like a pendulum movement running without a pendulum. Just very fast movement of the anchor. If it does not happen while the clock is running on its own, it's not a problem. If it does happen on it's own, the fork has to be raised a little bit. If the clock is only gaining 5 minutes a day or so, it's probably not fluttering. You would see hours rather than minutes, and you would hear it while it's fluttering.
A more likely issue with gaining time when the pendulum is adjusted fully slow is the suspension spring being the wrong thickness.
It's a shame Joe Rabushka's book is so hard to find. He's got a chapter dedicated just to the fork and its location on the suspension spring and it contains excellent information regarding fluttering. I agree with whatgoesaround - raise the fork on the suspension spring in small increments; low fork location has been the cause of the cases of fluttering I've suffered before. Good luck with it!