MacBook keeps forgetting network

Actually it is somewhat common that the symptom that you are seeing with the MacBook is because the AirPort credentials, saved in the OS X's KeyChain, are either corrupted or missing.

I suggest that you use the Keychain utility to remove all instances of saved credentials (keychains) for your TC. Log-off/log-on or (better yet) restart your MacBook; re-enter the proper credentials to access the TC; save them to the Keychain ... and you should be good to go.

How exactly do I save the credentials to my keychain?

A "credential," in this context, is another term for the combination of your UserName & Password. Keychain is OS X's password management system. Anytime you are asked "Do you want to save this password?" answering "Yes" will store it as a Keychain entry.

Try the following, in order, until (hopefully) resolved:
1. Delete Preferred Network(s)
o System Preferences > Network > AirPort > Advanced > AirPort tab
o Under "Preferred Networks," delete the network(s) you regularly use from the list.

2. Delete AirPort Keychain Entries
o Launch the "Keychain Access" application located in Applications/Utilties.
o Click on the "Kind" filter at the top, and look for any "AirPort network password" entries...and delete them.
o Restart, or log out then back in.

3. Add Preferred Network(s)
o System Preferences > Network > AirPort > Advanced > AirPort tab
o Add the preferred network(s) using the "+" button.
o Restart or log out then back in.

4. Run the Disk Utility to repair Disk Permissions.

5. Turn off the Mac OS X Firewall
o System Preferences > Sharing > Firewall
o Click the "Stop" button.

6. Move System Configuration Files
(Note: You will have to reestablish your network connections settings.)
o Go to /Library/Preferences
o Move the SystemConfiguration folder and com.apple.networkConfig.plist file to the desktop.
o Restart your Mac. (Note: OS X will rebuild the files that are now sitting on your desktop. If this doesn't resolve the issue, you can move the folder & .plist file back to it's original location.)

7. Create a New User Account
o System Preferences > Accounts
o Click "+" to add a new account
o Log off, then log back on with the new user account.
o Try connecting with the AirPort.

I have to say these instructions worked for me as well.

I've been on my own Time Capsule network for a couple of years with various system updates, upgrading to Snow Leopard, and using all kinds of browsers. Only just about 10 days ago (after an Apple update for RAW or something) I started getting an intermittent WiFi connection. I would be working away and suddenly there's no internet and no local WiFi. Funny enough I could bring my MacBook Pro closer to the Time Capsule and it would reconnect. Walk away some distance, lost connect.

I did a bunch of the usual things with restarting Time Capsule and other tweaks but nothing seemed to help. Some days yes some days no. I also suspected my own aging machine of having antenna issues with overheating (and other paranoid ideas about sun spots and stuff) when I read this simple list of checks.

Once I axed the Keychain reference, the Preferred Network, logged out and back, I had a solid connection.

Thanks for making such a plain and simple solution to a rather strange and confounding problem. Perhaps some day one of the system utility makers will add a kind of *keychain check* to their host of crons and things. But for now I can only add my endorsement of this solution.

Cheers, Marshall.