Radio/Speaker Wiring Problems

I recently installed four new speakers and a new CD player in my 1997 Explorer. Ever since, they have not worked properly. The left-front and right-rear speakers seem to be the worst of the bunch. They cut in and out quite often, only producing about 10% of the original volume. I have double checked all connections at the speakers and behind the radio and have even redone some connections, but to no avail. Can anyone offer any suggestions as to where my problem might be? Anyone having a similar problem? Please respond if you can help me out. It would be greatly appreciated. Thanks.

i would rewire them straight to the deck with some good speaker wire this may seem like a lot of work but it's guaranted to work. solder all your connections and use heat shrink then you'll never have to worry about the wires again.

Thanks for the suggestion amac209. I was afraid it might come down to that...