I had a strikingly similiar problem after updating to 10.50. I also have a GTX 770 (Asus), Windows 10 64-bit (with the latest updates). My frame rates would "very randomly" drop to 19.90 (and always that). I have a i7-4790 processor which appears to only run at 25-30% capacity with x-plane. I kept minimizing my settings, and able to get it to work better with medium settings, but it would then still drop again to that 19.90. Very frustrating ! I was able to run x-xplane on high with no problems before the 10.50 update.
Anyhow, I deleted all plug-ins and reinstalled x-xplane and was testing it without plugins. I ran Techpower GPU-Z against it, and it appears that the culprit is it is running out of VRAM.
It is a great card, but it appears that the 2gb VRAM is just too low given all of the world view updated changes, etc. in the new version. I was thinking about updating to a gtx 1060 card. I know, I know, but what are you going to do ? I planned on posting a question about this myself as a sanity check in the forum, and see what card is good I can get away with for the least cost. I only use my GPU for x-plane, and i like the 770, but my problem is the vram not being enough. It is a good card, but 3.5 years old, so time for a regretted update I guess.
Also, I have not used alt + tab in ms windows 10, or anything as mentioned in other answers that I think would be causing this.