60Hz screen not capable of 60Hz?

I finally realized that my IPS screen (Asus PA246) is the culprit with poor video playback of 60fps video. I thought it may have been some other, software related issue, as it usually plays @ 60fps for the first few seconds, then starts to have frame drops and stutters. Nearly all 60fps videos I try to playback on the 60Hz IPS screen, will have dropped frames/stuttering, while the 120Hz screen is silky smooth. Even the fps monitor in MSI Afterburner shows this clearly, with a solid straight 60fps line on the 120Hz screen, while the 60Hz screen has constant drops into the low 40s. Being the primary or secondary monitor doesn't matter, as it was doing the same even when it was the primary. I'll have to test it someday when it's solo, but I wouldn't be surprised if it does the same then.

Anyone else have issues like this?

Are you running a 120 hz and 60 hz monitor at the same time?

Yes, and it did the same even when it was a 60Hz secondary screen.