I'm with allen on this one....I myself was having the same problem so I took it up to the local Chevy Dealership and had the pressure control selenoid replaced.
The whole job cost me $320.
FOR ALL YOUR WHEEL, TIRE, AND ACCESSORY NEEDS
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2000 Chevy Silverado 4.8L RCSB: smoked UPD, Flowmaster exhaust, De-screened MAF, Nelson 93 hot tune. Upcoming installs: trans-go shift kit, Eaton posi, billet servo, ported and polished throttle body...Near future: 6.0/4L80E install with 150 wet shot