Home Networking Problems - Wifi Signal Sucks..How to improve?

I don't have a big house (1,500 sq ft). I have AT&T Uverse Gigapower now, and had Uverse before gigapower as well. Both routers used for those services had issues getting the wifi signal to all of my house. The signal is intermittent and unpredictable. It's incredibly frustrated because I have to switch wifi off and on a lot on my phone and other devices so that they are useable. I spoke with AT&T support and they thinkt here may just be an issue with my house or something interfering. I can't figure out what it is, but it seems that they are right because I bought a more powerful router at Fry's and turned off the wifi on the uverse router. I noticed some improvement but still have issues with connection going in and out while on wifi. I'm at my wit's end now.

I have every room in my house networked with CAT5 and I'm wondering if there is any device like a repeater or something that I can plug in to the LAN that will distribute another wifi signal on the same channel as the main one and on the same network (so you wouldn't have to switch between two wifi networks).

Any help would be appreciated.