Whirlpool ADP 4500?

We have a Whirlpool 4500. Its currently not cleaning the dishes very well. We believe that this is due to a part that repeatedly becomes dislodged during a wash cycle.

The rotary water sprayer on the bottom has just behind it a moulded plastic tube (2xtubes like a double barrel shotgun, which incidently I would like to use on this dishwasher) that tracks along the bottom towards the back and runs up the back of the unit to, we believe the top water sprayer. During the washing cycle this tube becomes dislodged. We put it back every time.

Does anyone know how this tube fits. It seems to snap into place solid but still comes out. Or can anyone tell me what it is called so that I can buy another. Could for some reason, the water pressure form the pump have increased forcing the tube out.