I experience the same issue as well and on the latest version (770).
I can upload between 300-600 mbps on every fresh reboot, and after a while, it drops to 50-150 mbps and later dropped to 0-50 mbps. (On a 1Gbps line)
The weird thing is that on every fresh restart, I can go back to 300-600 mbps for a while until it drops again. From the look of it, it seems more like a CloudDrive issue rather than Google.
Tried changing the thread settings etc and nothing seems to help.
Already reported to support and waiting for a resolution