1999 Toyota Camry hesitation

Last 2 years, engine hesitates most noteably when cold at 66 km/hr, but also at other times and speeds. It is extremely hard to duplicate on command for the technician. I thought it was the transmission and had it rebuild for $3600. It did not cure the problem. Two dealers could not properly diagnose or cure the problem. Over $800 already spent on this. There had been no engine codes until last week. Engine lite on for just one or two days and dissappeard. It produced code "P0446. Identification: EVAP system - vent control malfunction. Possible problem area: same as P0441. Identification: EVAP control sytem - incorrect purge flow. Possible problem area:
- open or short in VSV circuit for EVAP
- VSV for EVAP
- Open or short in vapor pressure sensor circuit
- Vapor pressure sensor malfunction
- Open or short in VSV circuit for vapor pressure sensor
- malfunction in VSV for vapor pressure circuit
- vacuum hose blocked, damaged, or disconnected
- charcoal canister malfunction.
The activation of the engine lite did not coincided with any other symptoms. Original symptoms remain the same. Dealer said aftermarket fuel filler cap could be problem. I had it for 1 year, well after symtoms started. Still makes hissing sound when opened for fill-up.
What is your guess of the cause?