Why does my computer have intermittent and weak wireless connection?

My computer has intermittent and weak wifi connection with any number of networks. My computer can show full bars, go to no connection back up to 2 or 3 bars and repeat, all in the course of 5 minutes or 3 hours while sitting in the same place, on any number of networks. Occasionally I will have connection but report a self assigned IP or no IP address at all. I can be in the other room or 2 feet from the router and get the same results. Running network diagnostics most often shows that the connection fails at the ISP level onward. When running diagnostic does fix whatever is going on, any network changes it notifies me of are not noticeable anywhere. Connecting to any given network via ethernet cable renders no problems at all.

I wanted to know if there is a definitive way to check the functionality of the different components involved (ie. antenna, airport, connections, etc...) or see if it's a software issue. I'd like to avoid a situation where I buy a new airport card only to find out it wasn't bad in the first place. If you have the answer already, that's cool too.

Supplementary information:

• MBP Purchased mid 2006
• 1yo replaced logic board
• Original airport card
• 1yo HD upgrade
• Screen hinges have been tightened several times in the past year.

Thanks in advance for the help. Sorry if there are any red hearings here.