Sporatic graphics lag

Details:
Ever since the July update (approximately), the game's graphics tend to stutter sporadically, sometimes for hours at a time. And then for no apparent reason, things return to normal. When lagging, the game is much harder to play. I've taken screenshots of what's going on in the Windows task manager, and CPU, Memory, Disk, and Network utilization all remain about the same between slow and normal times (~10%, ~60%, ~1%, and ~0% respectively). No background tasks stand out as a culprit. Still, I've uninstalled my corporate antivirus just in case, and that had no effect. The only thing that seems to differ between slow and normal times is GPU utilization. Believe it or not, the lag occurs when GPU utilization is *low* and goes away when GPU utilization goes up. The PlayOnline Viewer goes from ~24% GPU utilization during lag up to ~48% utilization when running normally. It seems as though the game isn't getting the GPU dedication it needs. I don't know if that's a game error, OS error, or driver error.

All the latest drivers are installed.

Steps:

1. Run the game - it may start with lag or running normally, and every few hours it will switch. Zone transitions do not cause a change.

Date & Time: since ~July
Frequency: Always
Platform: PC
PC Manufacturer & Model Number: Surface Pro 6 Model 1796 i5
OS: Windows 7 Professional
Graphics Card: Intel UHD Graphics 620 (integrated)
ISP: Spectrum
Type of Internet Connection: Cable (using both wired and WiFi)

It turns out that this was either a driver and firmware issue with the Surface Pro 6. I learned this by talking to Microsoft support. The problem mostly disappeared after a BIOS update I installed in early October, and I've been told a "system update" is also forthcoming to address the issue (as of 16 Oct 2019).