How to diagnose Time Capsule issues
The main issue you mention of network strength and/or performance degrading should be something you could catch with periodic dumping of the RSSI status using the airport binary. I sym link it to /usr/bin for $PATH reasons and ease of access, but it's really buried in a System framework.

Air:~ me$ which airport
/usr/local/bin/airport
Air:~ me$ ls -l /usr/local/bin/airport
lrwxr-xr-x  1 root  wheel  89 Apr  4  2012 /usr/local/bin/airport -&gt; /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport


If you were a unix sysadmin type, you would set up SNMP log forwarding on th AirPort/Time Capsule to send all the logs to your Mac and configure syslogd to file those airport messages in a log of your choosing. The Airport Utility version 6 (and the iOS versions) lack the ability to set a SNMP remote log host, so download an older version and maintain a copy of it going forward in case you want this extra functionality.

The --getinfo option will let you graph your observed signal and noise levels and you might even automate --scan to see if you can correlate drops to other networks showing up on the same channel with a better signal (or even causing excess noise). Once you've been able to catch 10 failures, you can start going over the logs to see if you can detect any patterns. You'll also need to keep good notes about what you change (and when and why) in terms of reboots, drops, changes to channels, etc...

Depending on how bored you are, you could script these data collections to append xml or csv and use a spreadsheet to chart the values over time. If you were supremely bored or motivated, set up MRTG - but for most home networks, it is severe overkill to configure let alone maintain.