To really dig into your problem, you could use an RF spectrum analyzer like YellowJacket to examine the output signal and diagnose sources of RF spectrum interference. Alternatively, you could use a WLAN analyzer like AirMagnet or WildPackets AiroPeek to measure signal strength and watch the protocol interaction that occurs when the association drops between your bridging APs. However, because you're using two identical products, you're probably not struggling with an interoperability or mismatched configuration problem. You can probably just quickly verify that your problem is poor signal strength by running a shareware tool like NetStumbler.
Place a laptop running NetStumbler next to one AP to view the current signal-to-noise ratio of their association. If SNR is consistently less than 20, then take steps (like using external antennas) to increase signal quality between your bridging APs. If SNR varies widely over time, then you're experiencing another problem like multi-path that requires further digging and might be addressed by repositioning APs or antennas to avoid obstructions reflecting RF. Links to these and other WLAN diagnostic tools can be found on my website's WLAN Tools page.