Is there a shampoo bottle on it? IF SO fill it up (with water if nothing else)
as it could be drawing air in from there (might be worth making sure the little pipe that joins the pump to the shampoo tank is tight and is actually in the tank.

make sure the water supply getting to it is good, and the hose isnt kinked.

if the pressure is still coming and going, then it sounds like the seals might of gone in the pump head.