games stutter or won't launch, could directx 12 be the problem?

dxdiag will tell you: Windows key, type dxdiag, enter. System tab shows the DirectX version, Display tab lists the driver and any problems found at the bottom. But DirectX 12 ships as part of Windows and can't be reinstalled on its own, so if dxdiag is clean the problem is the driver, which is what you changed. Roll it back: Device Manager, Display adapters, your card, Properties, Driver tab, Roll Back Driver. If that's greyed out, run DDU in safe mode to strip it completely and install the previous version from the maker's site. A fresh driver breaking DX12 titles while DX11 runs fine is a well known pattern, especially the first release after a big game patch.