Check Engine Light

[Jackd]

The car:
2000, W202, (C280) with 142,000km (88,000mi)
Check engine light is on, code: P0455.
dealer's diagnostic: Faulty gas cap..... replaced.
2 days later, the same check engine light is on, same code found (P0455)

Dealer says they'll do a "smoke test". Nothing found, everything is OK as per the dealer, but suggest I have the evap canister replaced.
The dealer clears the code, good for 2-3 days, Check engine light is back on again.
The light goes on only when engine is idling for a few minutes. First, the low fuel light would flash, which seems to trigger the Check Engine ligh.
Dealer now wants to replace the fuel pump......
Any hints before I let those incompetent throw parts at it again?
Thanks a lot