DF 140 loses water pressure

[William Moore]

Have a DF 140 that loses water pressure while running. Starts out on plane at @7 lbs pressure then few minutes later drops off to 1.5 - 3 lbs and sometimes to 0 then light comes on and motor shuts down. Have replaced water pump impellor and plate and still loses pressure. I can power down, put it in reverse and then power back up and pressure restores to normal then few minutes later drops off again. I check thermostat and is working fine.
Any help from anyone who has had similar problem would be apprecieated.
Thanks,

[boats.net]

Make sure all your water passages are clear from debri. There is also a water pressure valve, should be located on the block lower starboard side.