Motor

I have a Chevrolet Silverado 1500 2wd extended cab.
It has a 350 vortex motor, my second one  is a crate motor with heads. I forget
how many miles it has now but it up there, but it has done it every since I
installed it the original one did the same thing. My question is when I first start
it, it has 40 pound of pressure but then when it warms up to temp it reads
almost 0 when I come to a stop, then when I take off it will read 15 to 20 or less,
then when it cool it does the same thing.