In my experience this is the pin-cushion effect. Please indicate which sources you used to modify your CC video as most have a very detailed adjustment section, which is all this should be. I have performed two different methods of the video mod and in both cases had to make substantial adjustments to the CRT. I do not recall if all were controlled by the rear controls or if there were others as well. Check the http://colourclassicfaq.com/ which should have the adjustment details for you to double-check. This and many other sources around the net should provide all the information you need to thoroughly troubleshoot common issues.