Another Update:
Despite replacing the Gas Cap and Purge Solenoid - which for some reason worked for a while - the GASCAP error came back with a vengeance. Finally crawled under the back end, removed the EVAP canister (ok, who's idea was it to put this thing back there up above the axle??). Finally got the canister out...hoses seemed fine, cable looked fine. What I found after removing the white EVAP solenoid (or is it the NLD Pump??) - it twists off - was that one of the connector contacts had corroded away in the gap between the connector housing and the solenoid housing, hence a fault whenever the PCM did an EVAP test (no power to the solenoid). A Dremel and soldering iron made the repair to the contact, now no error (crossing my fingers tho). Pretty obscure failure, but beat buying a new canister for $90.