Justin,
I know exactly what you are going through. A lot of people already mentioned possible problems. The biggest one that I find myself doing now is not following through or having a weak follow through, especially on my spare shot.
Another as howie mentioned, is muscling the swing, which I noticed was happening when I didn't get the weight of the ball on my left hand before I started and during my push away (I'm a righty). Also it occurred occasionally when I would and still sometimes do, cut off my back swing (I'm working on a free swing and having some success, but you know what they say about old habits). You might also want to check your fit to see if it still is good, lest you end up like me and you have to put 5 pieces of
tape in your thumbhole until you can get fit fixed up.
Something else that I noticed is that when I'm rather lax when it comes to using my balance arm and trial leg I tend to be inconsistent as well. I'm a big believer in counter motion. I mean it is a very important concept in two of my favorite activities, bowling and my martial art, Hwa Rang Do. When I'm lazy with my balance arm, trial leg, or both I can't really get into the comfortable and consistent position to perform well. It might be something that you could look into.
Lastly, it could be something as simple as fixing up your release. That's something I've been working on recently and its been a little tough but I'm getting there. Additionally, it could be something like your wrist can't support the weight of your bowling ball.
The best way to find bad habits and things to fix is to get yourself on
tape. I find lots of things that need correcting when I videotape myself and critique myself. I made the decision to switch back to a 4-step based on watching my performance on
tape. It could be very helpful and one of the best things you. Then after you have critiqued yourself, get a second opinion or rather a lot of second opinions via the rest of us on the forum. Each of us has a different perspective so we might find something another person my have missed. It couldn't hurt. I also decided to scrap my entire approach based on the comments of the folks here on the forum and I feel that I'm better off because of it despite the rough patches.