When i'm going uphill or pedaling really hard, something slips...and i think they're my cranks. Anybody know how to fix this? Also...does anyone have any recommendations for a crankset/chainring (don't know what if im going to replace just the chainring or the whole crankset yet) under 70 dollars for a Motobecane Mirage (1980's). Thanks.

define "slips"

it could be that your crank is wearing out and developing some wobble on the BB spindle. it could also be that your cog and lockring are not quite tight enough, causing the cog to have some play between the lockring and the hub wall. it could also be a chainline issue if there's a little "pop" to it, or a bent tooth, or a worn out tooth.

i think it was the lockring. thanks

Your cog is probably slipping. Have your hub looked at by someone who knows what they are doing, it may be shot/damaged, or might just need both cog and lockring properly tightened.