Why is my iPhone charger not working properly?

So, I put my iPhone 4S on the charger hooked up to an HP computer. It will work okay for literally 10 seconds, then it tells me that it doesn't have a charger in, then it will vibrate to notify that it's charging again like half a second later. It's slowly losing battery life and it's extremely annoying. I tried it with the wall charger, and it was messing up there, too.

I get this charge on - charge off thing when I barely move the phone. This is seriously irritating me like no other.