Pull the shutter back and have a look at the disc surface under good light. It’s common for mould spots, especially if they’ve been stored in a loft etc. Can be cleaned gently with cotton bud and deionised water. Or, maybe one very bad disk has dirtied the new drive heads, and a clean would help.
Thanks, yeah the disks are from the 90s so might have collected some dust and mould over the years. Will give this a try.
Good luck, I hope it helps! If it does seem to be improving things you can get 3D printed tools to hold the shutter and turn the spindle. Makes it a *lot* easier if you have loads of disks to do! I got mine from RMC/The Cave but a bunch of folk sell them online now.