1) Check your battery connections. They need to be clean and tight. Side posts are terrible about backing out on their own. You replace the battery and suddenly it works great again, but a week later the post loosens up and you get an intermittent no-start, or even the truck shutting off while you're driving down the road.
2) Check your battery connection to the starter. It also needs to be clean and tight.
3) Check your grounds. Clean and tight.
4) Check your alternator connections. Again, clean and tight.
You've stated that the battery gets weak and that it sometimes cranks slow. I think your old battery was fine, but not getting charged or not making a good connection. Your alternator could be weak, but make sure all your connections are solid before you start testing the alternator. It won't charge if the connections are bad. Your problem is not the starter, the ignition module, the steering column, nor ignition switch.