Please read through my link before you do anything else. You have a tuning/equipment issue and mechanical heads won't fix the problem. In fact they could compound it. Bow tuning comes first, then you can decide what broadehads are best for your setup.
http://www.broadheadtests.com/TUNING.html

Your bow poundage is too light to even consider using a mechanical. If you do the results would be the same as your handle... Bad Shot.