Still stuttering, hanging sound and missing textures with ASUS Strix RX480 OC 8 GB since 5 months

Since a few months my Forza 7 Ultimate is still unplayable for me.

As the topic says, I have massive frame drops, missing textures on track (they show up very lately) and massive stuttering. Sometimes the game hangs in a race for 5-10 seconds and the sound loops than. It doesn’t matter, what resolution and details I choose. A few months ago (before August update), I was playing in full HD ultra and with 2x MSAA @ 60 fps V-Sync.

What’s the matter of this now? Is it still an AMD driver issue? I have newest AMD drivers installed. Is it a Win 10 update issue? I’m still running 1803 x64, but I have all other updates installed. Is it a Forza 7/engine issue? I payed over 100,- € for a game, that I can’t play for months now…

If somebody has a solution for this, thanks in advance. If not, maybe I should open a support ticket?

Edit: in benchmark mode there are no GFX issues. but there ware also massive stuttering and it says minimum FPS-rate 29.9 fps…never had this minimum low frame rate 5 months ago.