Why are my halogen bulbs dimming, then restarting.. then dimming?

Since I bought my place, the MR 16 halogen bulbs on my tracklight haven't worked right.

I turn them on, they light for light 5-10 minutes, then blowout.. .but they're not really blown out, because if I shut them off, and turn them on again later, they'll work.  Sometimes they'll just go back on by themselves.   I've bought new bulbs, replaced the little sliding arm things they plug into...  only put one track light at a time up... nothing seems to work.  I don't get it.