FINALLY, AN APPARENT RESOLUTION!!! WE HAVE BEEN BARKING UP THE WRONG TREE AND HAVE LED OURSELVES ON A WILD GOOSE CHASE!!!
For some reason, the New Blue Color Fixer Pro plugin has caused all of the anomalies that I reported experiencing when used in ANY version of Magix Vegas Pro in which I tried it (VP 18, VP 19 and VP 20). Having stumbled upon this as a possible source of the problem, I substituted a similar Boris Effects Plugin in the place of Color Fixer Pro and have not experienced any problems whatsoever! Eliminating Color Fixer Pro apparently has resolved the issue!
I have regained FULL use of my graphics card and set the RAM Preview at 50% of my installed RAM. No problems editing or rendering (at full speed) during the past half dozen or so editing sessions!! Knock on wood (LOL)!
Thanks again to everyone who chimed in to try to help, and I’m sorry that you all wracked your brains out. So the moral of the story is ALSO to check out if plugins may be the culprit when such anomalies arise BEFORE jumping through all of the hoops that I had to go through! What an educational and learning experience, though!!
Thanks again!!!!