Single pot light in room continually dimming / flickering, before dying after a few days
I think you know the answer. You had a 50 watt lamp in a fixture rated for 42 watts. 

The cheapest solution would be to have used a less powerful bulb. But it's possible that you have damaged the fixture. I think you should try harder to find a less intense bulb. I just shopped online for 1.37 seconds and found six MR16 type bulbs rated below 50 watts.

Or you could change the fixture. You might need a new one now, anyway.