Monorail Track Lighting

Help with Track Lighting dimming issue, fire hazard? My monorail track lighting does not turn on immediately when I flip the switch, The lights do turn on after a slight delay, but they will begin dim on their own and eventually go off again after some time (perhaps 20 min ). They will go on again after a while (maybe after it cools down?) This started about two months ago when I replaced two light bulbs. I have taken the bulbs out again, but it does not seem to make a difference. What is causing this, and what can I do?