My '96 Hyundai Elantra 5 speed hesitates from 1st to 2nd gear after it has warmed up..?

I have changed the fuel filter and added 'Chem Tool " fuel treatment . Other than that the car runs great. Also when the car idles for a extended amount of time it reeks like rotten eggs. Could cheap gas be a factor?The hesitation and loss of power are not a constant but happen enough that I can't tell when it's gonna happen ( my poor clutch), . The car idles and starts fine.The power loss happens after the car has warmed up, never cold. Thank you for your input.!