07 Elantra problem
 

I am having an issue and of course can't get it to duplicate when at mechanic. Check engine light is on for a P0444 code. I live in FL and sometimes in afternoon i am getting gas fumes in cabin while driving. I am also getting intermittent power cutting out at a constant speed. Mechanic has taken car for 2 days and used it to run errands, go home and even left running one afternoon to reproduce problem and nothing happens. We have changed purge control value under hood and he even ran leak test to see if any leaks in fuel system
Car has 228, 000 miles and really wanted to keep for awhile yet but worry of break downs. Any help??