I would check proximity of your Expresses to your TC. Even when attached via Ethernet your Expresses must be within range of the TC or they will loose communication & disassociate. The network traffic may run across the Ethernet but they must be able to see each other over wireless in order to successfully negotiate clients to move from one to the other, so if they loose the ability to see each other over wireless the Expresses will flip out. This has been my observation anyway. The expresses are also extremely weak in comparison, I have not found them to be much use as far as range extension but they are handy when you want to force N on your time capsule & then keep the Expresses for your slower b/g devices. Your wifi is only as fast as your slowest device so even if your laptop says connected at 140 or even 300Mbps an older Mac connected at 54Mbps will slow your entire network down to 54Mbps.
No, the Airport Utility actually is a piece of cr*p. Basically anything that's wireless from Apple has been a major pain keyboards, mice, airport express and time capsule. All need restart after restart. Range isn't really an issue in our 41 square metre flat but the devices wigging out regularly is. Apple's wireless departement works like Corel "It isn't stable? That's ok, we'll give them a couple of new features and they'll forget about it."
Basically I can sum my experience with Apple in this: "Have a wire? Things irie."