No video playback in sequence, rendering with frozen frames

My setup:

iMac (Retina 5K, 27-inch, 2020)

3.6 GHz 10-Core Intel Core i9

128 GB 2667 MHz DDR4

AMD Radeon Pro 5700 XT 16 GB

Was running tests with different export setrtings trying to overcome some issues with color and encoding noise, project seemed to run fine the first few times but now there is a yellow bar over the entire sequence and the clips that have speed ramps are now freezing!  

was also suprised to find that harware encoding is NOT available for H.265 with my GPU...i can't believe it actually...

Can a support member please reach out asap!

-Jay