From one match to the next my ping can go from a manageable 60 up to a virtually unplayable 200. What gives? This is even on my desktop that has an ethernet connection to my router, so it's not just a wifi thing. I don;t want to do anytrhing drastic like change my ISP or buy a new modem or router unless I know for sure that will fix the problem. But doing nothing will also obviously mean some online matches end up being frustrating arse kickings.

My base internet speed is a reasonable 50Mbs download, not sure what the upload speed is but it will be at least 2Mbs. Normally the speed remains pretty consistent.

What game?... Or is it multiple games this is happening to?

It's not drastically changing from match to match, it's drastically changing from moment to moment. Call your cable company, perhaps your wiring is going bad. I had dips yesterday down to 1.5MBPs, and they came right out today and fixed it in about an hour.

It could be something else too, though.  Just... run some tests, call your cable company, and talk to them, and see how it goes.

Try to debug. Ping your router (ping -t) and see how it goes. Then ping the next “hop“ which you can find out via traceroute to something on the internet. It should be the second “station“ - it should be a router from your isp and see how this goes.

Dedicated servers are a myth.