I personally find the interlocking grip much harder than the overlapping grip to get comfy with. The reason why I am so inconsistent with my shots are because I find it hard to know what I've done wrong when I hit a poor shot.
I also find the interlocking grip difficult, I have used the overlapping grip for as long as I can remember.
With regards to working out what's going wrong when hitting a poor shot, have you taken lessons before? Having the theory behind your golf shot will help a lot with this.