Random Display Color Pattern on Shutdown

Hi.

This is a brand-new MacBook Pro with Touch Bar (more configuration details below). Suddenly, for no apparent reason, during shutdown and restart the display shows a random pattern of colors on a black background (see attached image).

I have reset the NVRAM and the SMC as advised (or think I have -- not sure how to tell if they were successfully reset), but the problem persists. The pattern varies from shutdown to shutdown. I am running an external Dell monitor through an OWC Thunderbolt dock using a Thunderbolt-to-USB-C adapter. The color pattern appears on the MacBook Pro screen, not the Dell monitor, but the Dell monitor shows the cursor pointer for as long as it takes the computer to shut down. The problem may have begun around the time I started editing video with Adobe Premiere Pro, but I don't know if this is relevant or a coincidence.

Any help would be much appreciated.

Thanks,

Paul

Computer Hardware: MacBook Pro (15-inch, 2017); 2.8 GHz Intel Core i7; 16 GB 2133 MHz LPDDR3; Graphics: Radeon Pro 555 2 GB, EFI Driver Version 01.00.927 AND Intel HD Graphics 630 1536 MB, gMux version 4.0.29 [3.2.8].

OS 10.13.1 (High Sierra)

Peripheral Hardware: OWC Thunderbolt 2 dock (Thunderbolt 2 | USB 3.1 Gen 1 | FireWire 800 | HDMI | Gigabit Ethernet | Audio In & Out), with following devices:

DAS Keyboard 4 Professional for Mac

Dell U2412MB External Monitor

G-Technology 4TB G-DRIVE G1 USB 3.0 Hard Drive