I have the same problem with Ralink RT5390 on a HP dv7 laptop.
Everything was working fine with ubuntu 12.10, but after upgrading to 13.04
and later to 13.10, the wifi was continuously connecting/disconnecting and was very slow.
Hopefully, I had not removed my last 12.10 kernel version (3.5.0-28-generic),
and found that if I boot with it the wifi is working fine.
The kernel versions 3.8.x used with ubuntu 13.04 and 13.10 seem to have the problems.
Also, when using kernel 3.5.0 to boot the wifi on/off button is working,
while when booting with kernel 3.8.x this button is dead.
From time to time (i.e. when new 3.8.x kernels are installed during ubuntu updates) I check if things are ok,
but so far this is not the case and I am still going back using the older 3.5.0 kernel.
It seems that something was broken for the ralink driver rt2800pci which is used
going from kernel 3.5.x to 3.8.x and nobody has ever fixed it.
I have ubuntu 13.10 / Ralink RT5390 / HP dv7 and use the the rt2800pci driver (as it comes after installation)
kernel version WiFi status WiFi button 3.5.0-28-generic works well OK 3.8.x slow with many disconnections not working