Hello @Top Bloke (AUS)
Have you got any background programs open, such as Google Chrome, Music Player or any other programs which takes a lot of resources of your Pc? If yes, please try playing without having this programs open. Also go in your Steam Library and check in Properties at Euro Truck Simulator 2 if your Files are all not corrupted, this can also cause that you have FPS drops.
Another option is that your telemetry is corrupted or your Client for saving your Loads like TrucksBook or SpedV is broken/corrupted.
If this helps , let me know if that solved your issue with FPS Drops. If you still have this issue also contact me i will be willing to help you in any problems/issues you are having in game or otherwise. Kind regards
Ps: I have found a link which can be helpful for you: