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@qckpckt
you need a new crank arm.
Basically, the crank is held onto the spindle by the pressure exerted by the bolt. This only works if the taper of the square hole in the crank accurately corresponds to the taper of the spindle.
If the bolt isn't tightened enough, or if it works itself loose (does happen every now and again), and the bike is pedalled, this deforms the tapers on the crank (because the metal is softer than the metal of the spindle.
Once this has happened, no matter how often you tighten the bolt it will work itself loose again, basically because the metal simply gives way and moves further up the spindle towards the bike frame.
Dunno which shop you went to but it's got nothing to do with anything being cracked. You want a new crank arm. left hand ones are fairly readily available, for this very reason. Any respectable bike shop should have them in stock and as long as you have the bolt the swap over procedure is fairly quick and painless.
Hope this helps.