Taking out CLS made no difference
Thought not, but as it is superfluous it is best to eliminate it from the list of possibilities (because it was in a non-standard place and drivers can be odd).
You really, really need to vsync to get a satisfactory image though. It's no good matching the monitor's refresh rate exactly if you're sending it each frame when it's, say, half way through its scanline cycle. Unless you sync, it's pot-luck as to which scanline you'll catch for the duration of your program.
Welcome to the forums, BTW. The issue of updating the display is a particularly thorny one on a platform like the PC, where the hardware is not fixed. Once you get used to Blitz you will find that there are other avenues to explore, such as so-called "delta timing" (personally I'm not a fan, but others swear by it).