Surging pressure at idle

[85 Riviera]

Trans is a 325-4L,
The gauges needle kinda moves or wiggles a small amount about one wiggle per second at idle in D, as soon as I barly lay my foot on the peddle the pressure raises fast and high as normal.

Now here something even crazier, when I put the trans lever in 1st or 2nd the pressure only raises just a tad higher compaired to D and it still wiggles but as soon as I lay my foot on the peddle the pressure raises like normal and low boost seems to be working then, I wonted to know if this was RPM related or TV related so I put my car in 1st and drove to about 25MPH then let go of the gas and let the car coast to an idle, the pressure stayed high and steady all the way down to 900 RPMs as soon as the RPMs went under 900 then the neddle would start to wiggle and the pressure would drop again,
The cars Idles smooth and the trans performs great also the car idles in N at 800 and in gear at 600 just so you know.

What does this sound like, Its been about 4 months since a filter and fluid change and the level is on its mark, Im confused,
Thanks in advance!

 

[85 Riviera]

Well today I invested in a new filter and some fluid, I'm thinking the filter or at least hoping that the filter might be pulling in air or not sealing good to the pump, I wont be able to do the job till thursday, but anyway I am waiting for someones elses insight,

 

[chris718]

SOUNDS LIKE PUMP VOLUME AT LOW RPM IS LOW.this can be caused by a restricted vent on the transmission not allowing atmospheric pressure to fill the vaccum created by the pump fast enough as it can with the increased pressure drop available at higher pump speeds .this is acceptable and common with many auto transmissions .worn rotor also can do this as decrease oil can mix eith the pressure side of the pump.