i'm a new cycler and bought a low end 18 speed Pacific bike. when i pedal the chain comes of the rear derailleur. it will go off and come back on. i don't have to stop riding but it's not good. also shifting stinks. i shift the left and right and only seem to get about 3 gears. right side is numbered up to 6 and the left side has lots of lines that i can click into. the user manual covers every bike made by pacific so it's not very helpful. how can i fix the derailleur issue?