OK here is my prob..

We live in the middle of nowhere so we use tank water which is pumped into the house via a electric water pump, about a year or so ago we had solar hot water system installed and everything was fine till around a month ago now we get a loud knocking noice coming from the laundry(below the solar panels and tank on roof) and when the shower is running the pipes knock and the pump seems like it detects pressure(as if a tap was turned off) and the pump stops then starts up again at which point the nice hot shower your having turns into a cold shower for a few seconds the come back hot... basically during a shower about every 20-30 seconds we loose pressure the shower goes cold then back to hot(VERY ANNOYING) :(

Is there a simple fix for this if anyone has an ideas...

I should also note that there is no air in the lines in case someone was thinking that :)