i have a problem with my pedals and its getting worse. it seems that when i press harder or when peddling up hill its like my pedal or crank wheel slips. i have no idea what it is or how to fix this and would like to know. when it slips it makes a crackin sound like something popped and my foot always slips of the pedal. like i said i dont think its in the pedal it self but this occurs when i pedal my bike hard and apply alot of pressure like when standing up and peddling. anybody know what i mean?