With fear and trepidation I answer your question, because there is always some smarty alec, with more brains, just waiting to rip your ideas to shreds, but here goes.
It could be due to the ohm rating of the new cable, ie the new cable has what might be thought of as more signal resistance. Therefore weaker signal at TV and poor picture. You could try a signal booster.
Another fix, which even an ex Granada engineer could not explain, is, if the TV aerial is accessible, remove the shroud side di-pole from the clamping screw and either tape it up or bend it out the way a little so that when it rains water can't bridge the gap. We've had to use both 'fixes' before we could get a good signal.