On certain websites when scrolling up or down screen continuously jumps back to same point and same ad

1. Disable all Extensions and test.

Safari > Preferences > Extensions

To uninstall an extension, select it and click the “Uninstall” button.

2. Safari > Preferences > Search > Search Engine :

Select your preferred search engine.

3. Safari > Preferences > General > Homepage:

Set your Homepage.

4. Use free Malwarebytes Anti-Malware for Mac/ AdwareMedic to remove adware

https://www.malwarebytes.org/antimalware/mac/

Download, install , open, and run it by clicking “Scan for Adware” button to remove adware.

Once done, quit Malwarebytes Anti-Malware.

or

Remove the adware manually by following the “HowTo” from Apple.

http://support.apple.com/en-us/HT203987

5. Hold the shift key down and launch Safari

First, never use any kind of "anti-virus" or "anti-malware" software on a Mac. That's how you cause problems, not how you solve them.

Has this just started happening on many websites where it didn't happen before?

From your Safari menu bar click Safari > Preferences then select the Privacy tab then click: Remove All Website Data then quit and relaunch Safari and try scrolling on any website.