This has been discussed several times on the forum. It happens with the PMDG only, because the plane is simulating some kind of external force at all time acting on the nosewheel.
Which is entirely fine, they are probably enforcing a real force over the nosewheel, so the two systems (GSX and the PMDG) are fighting each other to set the same variable.
In the REAL world, to prevent this, there's the BYPASS PIN, which is inserted in the nosewheel to disconnect any airplane system from the gear, so the pushback truck will be able to rotate it freely.
And of course, GSX provides airplane developers with a way to check if GSX is pushing, using a very simple variable they can check in their code so, they can momentarily turn off whatever system simulation they have acting on the nosegear, simulating the effect of a real world bypass pin.
FS Labs, for example, uses this system, and GSX is even able to interface with the precise nosegear movement of their own animation.
Someone from PMDG promised a while ago that, after the release of the 747, they would have a look at this issue, and we got in touch about this. We shared a copy of our products (I sent two copies of GSX to them, they sent me a copy of the 747 to look at), but then, it seems we had some kind of communication misunderstanding, not sure if it was related to a language barrier or what, but at a certain time they just stopped replying to emails.
Unfortunately, there's just nothing we can do from our side to fix this. Only airplane developers can possibly know what's going on in their code so, the safest way to do it, is THEY would check if GSX is pushing and, do whatever they require on their systems, which is basically simulating what in the real world is the effect of a bypass pin.
Well, in fact, we COULD prevent the stuttering but, by doing this, it will also result in a fixed nosegear, not turning with the towbar so, I'm not sure which is worse, the stuttering, or the stuck nosegear.
That's why the proper way to fix this is from the airplane code (so only PMDG can do that, just like FS Labs did), because in that way, both the stuttering would be fixed AND the nosegear will turn with the towbar.
