I know your problem
It's the thermastat! If your hoses and stuff are at only 130-150, once you blow cold air across them the temp drops alot= luke warm air
My 99 Expy, I put a themp probe in the vent, and with my auto set to 90 it blows 164 out the vents!!
With you stat full open, the water isn't getting heated up enough before it's getting cooled down again... it's flowing through the block too fast! I bet if you look at your temp guage it's on the low side as well.
Drain the fluid again, take out the three bolts that hold the stat housing on top of the intake hooked to the upper hose. Pull off the housing pop out the stat and I just about bet money that it's stuck in the open position.