I went though the same thing with our Bramble. We tried all sorts but as the vet suggested she grew out of it for the main part.

She still had attacks which are stress related if leave her alone (with either Steve or a petsitter and 7 other cats )

Every poo attack we do the same routine, starve her for 24 hours with just drinking water and then feed her light food boiled rice & chicken and back to a normal diet.

The 24 hour without food rests her guts to allow good bacteia to build up again.

Good Luck (I know it is not pleasant dealing with these problems)