I have a bike which I use as normal as well as cycling indoors with my tacx flux smart trainer (direct drive, take the back wheel off). Without the trainer, my bike runs smoothly with no issues.
When it's connected to the trainer, it makes a clunking noise then it feels like the chain slips (the tension loosens, like i'm pedaling but the back wheel isn't moving) and it goes 'clunk, clunk, click' and after the click it rides fine for a bit then I have the same issue again. Sometimes this happens under high load but not always, sometimes it just happens when cycling at the same level. Whilst it feels like the chain is skipping a gear, it isn't. I've filmed it and it doesn't move.
I've bought a new cassette for the trainer, it has the same number of gears (9), however, it has 29 teeth instead of 27 on the largest sprocket. I was told this wouldn't be an issue.
I have tightened my cassette as hard as I can and it isn't loose when I shake it around on the trainer.
I have checked the derailleur and it is aligned perfectly with the cassette and the gears shift without issues.
I have taken the free hub out and put some lithium grease on it but with no luck.
I have measured the chain at a bike shop, they confirmed it was not stretched.
What can I do next? What could it be?
Videos of this happening,