sound like you just got the servos/speed control plugged in the wrong spots on the reciever.

try switchin the plugs for 2 and 3 on the reciever and try that....

also I would take anything that those people at that hobby shop with a grain of salt as it sounds like they just wanted a quick sale..

BTW. the futaba 9c is an excelent radio. i have one and I fly my hanger 9 j-3 cub, nexus chopper, and I used to fly my trainer (rip) all on that radio...

are you flying at a designated field? if so have them check things out before you try the cub... could save you a few bucks in repairs