Flickering Time

Riddle: With time I'm gonna flicker, only growing bigger. I'm handy during a test and do well with some rest. Sometimes I'll vanish without a trace, something's out of place. I come back with taste, I come back with smell. I'll give some of you hell, wishing I'd disappear and you go back in your shell. I can be tricked, I'm with you all, the poor and the rich. What am I?