Someday, Elitre should remind me to write up a standard help page on how to do this.
Here's the general system:
- You have some user scripts enabled at some wikis.
- You probably have some gadgets enabled in Special:Preferences at some wikis.
Gadgets are just user scripts that are easy to enable. But, as a rule of thumb, gadgets tend to be better written and better maintained than user scripts (also, there are wikis that don't offer any gadgets), so we begin with the assumption that the problem is in your user scripts. I'm also assuming that you've not changed any of your scripts recently, as you probably wouldn't be asking if it worked fine, you made a change, and it broke immediately afterwards. ;-)
So step #1 is "Find your user scripts". Here are your options, assuming that you're experiencing this problem at the English Wikipedia.
(There are also matching CSS files, if you have weird display issues, but that doesn't seem very relevant in this instance.)
Step #2 is to figure out which script(s) is the likely cause. Assuming that only one script is broken (usually true), then the fastest system is to blank each .js page in turn, and see whether that fixed the problem. So blank vector.js, reload, and see if the cursor still jumps. If it does, then revert your blanking, go to the next page in the list, and repeat.
When you discover which page seems to contain the problem, then we reach Step #3: Restore half the contents of the broken page, and re-test. Continue restoring (or removing) half of the scripts until you've restore all but one script.
The point behind doing this by halves is that if you have a lot of scripts, then it takes too long to restore each individually. A binary search tree is more efficient.