I have about 16 inches of braid line running from the tank over to the electric fuel pump, then connected to the hard line up to the mechanical pump. At first I had a cheap $13 dollar mechanical pump on it. Then I installed the electric fuel pump, then last week I installed the new Holley mechanical pump. And yes the problem only occurs when the fan comes on. As long as the temp doesn't get to 180 the pressure stays at 7 psi. However once the fan kicks on at 180 and hot air begins to blow the psi drops down.
Even if the mechanical pump was the problem, wouldn't the electric pump still produce a constanct flow?