how can i fix my worcester 24i combi boiler?

Because it's doing it on the cold water as well it's probably nothing to do with the boiler.
It's more likely to be the cold water stop tap vibrating when you turn the taps off.
Try turning the stop tap off and then turn it back on full. Now turn it back half a turn so it does'nt seize.
If that does'nt help you might have a non-return valve or a governor on the system that's interfering..
It's worth a try..

T.h.s
July 2007