Arthritis over time can damage joints. Within the joints are proprioceptive nerves that signal to your brain, your position, weight bearing and other information. So with continued damage, balance may be affected. Many people with arthritis may develop neuropathy secondarily due to damage of nerves in the joints or tendons.

Some drugs also cause neuropathy. There is new evidence out that many drugs may damage the mitochondria in cells. When this happens the cell can no longer "breathe" or create energy for itself and it stops working and may die. Here is a new post on mitochondrial damage and neuropathy:
http://neurotalk.psychcentral.com/thread122889.html
I am just starting that topic, but already finding alarming data on drugs and damage. I even found some reports that acetaminophen (Tylenol) over time damages mitochondria!

I think getting an EMG for the limb that is giving out, will reveal if there is axonal damage to the nerves. The inflammatory types of arthritis can also send inflammatory cytokines, to attack nerves as well. Some conditions like Lupus, can damage both nerves and joints. Any time a limb gives out, it should be evaluated by a doctor.

I am sorry this is happening to you, dllfo. I hope you find answers soon.