i agree with gucciman but that is only a temporarily quick fix it will happen a gain and again ,you need to find out what size the cold feed it might be 22 copper feed and might need 28 mm cold feed.
Or the restriction it could be scaled up inside and blocking the cold feed inlet to the cylinder,so when you draw of hot water to the eg to the bath the cold feed is restricted and you are emptying the tank quicker than you are filling it ,which will pull in air and your bath will trickle out and cough and splatter