I have a SCART lead that came with a QL that i bought. When connected to my LCD TV, it produces exactly the image that you describe (and later show in a photo).
I bought a new SCART lead from this eBay seller: retrocomputershack, item number 260814300633. It works great with my LCD TV, I get a nice sharp and clear picture with no noise or vertical white lines

Some (especially modern-ish) TVs, have automatic input detection. This uses the control signal pins on the SCART connector.
Some QL SCART leads use some of the signals in an unconventional manner to try to get the TV to automatically switch to the RGB mode, but instead the TV switches between composite video (with no input signal) and RGB video at high speed.
If your TV has manual / user controls that can override the automatic system, then manually selecting RGB mode may help with the problem.
Mark