Air Conditioner Mystery

This is driving me nuts.

I have 3 air conditioning units. One seawater pump services all 3 units. There is poor seawater flow from the overboard discharge from the forward most unit (the pump is in the stern). This unit is the furthest from the pump.

Here's what I have done to date (not in order):
Replaced the distribution manifold with new
Replaced the original pump with a new pump---1/3hp Oberdorfer commercial duty (oversized according to Depco)
Flushed all lines with Barnacle Buster=no joy
Changed all water lines with new to all 3 the units
Replaced all 3 AC units with new
Replaced overboard discharge shut off ball valve with new on forward unit

At times the forward unit discharge will be fine but after a few hours it will reduce to a dribble while the other two discharges are blasting out water at full flow. If I shutdown the pump and wait a few minutes then turn it back on I will get full flow at the forward unit again. After awhile it will slow down to a dribble.

My next and final attempt will be to change the discharge hose from the forward unit with new and remove the discharge ball valve completely. Then everything will be new. Please help before I need to take Prozac or throw myself overboard.

Greetings,
Mr. hm. Well, IF the only thing you haven't replaced is the forward unit is the discharge line, by process of elimination, THAT is the problem.
Might be the line has deteriorated to the point that the innermost lining of said hose has become detached from the body of the hose and is collapsing or is being forced closed over time (WAG).

or maybe an air leak that fills up the forward line and is killing the flow.... can you splice in a section of clear line or if it is...any bubbles?

I agree that the hose may be coming apart internally. If you replace the hose and it still has the problem maybe buy a new boat...