Thanks for your thoughts on this.
I restarted my PC and tried the default Cessna 172. That was smooth, no stutters.
So it must be the PMDG. Next I removed all WASM files related to the 777 and rebooted again. But I still get the stutters.
Really weird, I never had that issue. I think my next step is contacting PMDG support, unless you have any other suggestions?