1997 Dodge Caravan bad performance

I have been having this recurring problem with my 97 dodge caravan. The engine light has always been on but never really had any issues with the vehicle until about 3 months ago. When I start the van it starts fine at first but turns into a really rough idle, like the engine wants to die. When I put it into reverse the engine chugs and doesnt really want to take me anywhere. Once I get going to a higher speed, the engine hesitates as if the tranny is slipping. There is excessive gas coming out of the muffler, with an awful smell. At first when this would start happening I would use gum-out in my gas tank to clean the fuel injectors, ( that is what I was told it was, that or the O2 sensor) which would work for a while, the van would run perfectly. Then when it started acting up again I would do the same thing. Now that doesn't seem to be working at all and now the van is stalling and has a hard time starting back up. I was just at the mechanic and he used a fuel system cleaner to get rid of dirt and grim within the fuel lines and such, but now it seems to be acting worse. HELP! I have no idea what to do !