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Re: Flickering...

Dave Gomberg sez:
>Listen to the man!   Pattern.  On for hours (heating 
>up), off for 20 minutes, repeat.
>There is almost certainly a thermal shutoff in the 
>ballast and after two hours it gets too hot to continue 
>and stops.   After 20', it has cooled
>down enough to reset and starts up again.   Try cooling 
>the whole affair down a bit 

The lamp also gets hotter, which raises its internal pressure and thus raises its arc voltage.  If the ballast voltage is marginal (as was previously suggested), this would cause the cycling.  The lamp may be someting exotic and has a slightly higher voltage requirement than the ballast.  BTW, we don't even know if this is a electronic ballast, or CWA transformer, or maybe the ballast is designed for 150W instead of 175W, or...  

There is nothing obvious about this problem with the limited information we have been given.  The lamp could be at fault, or the ballast, or the capacitor (I guess that's really part of the ballast), or the socket, or the fixture wiring, or the power "mains" supply.  You are on the right track with heat being a factor, but what is getting too hot and why is that causing a failure?  The lamp is supposed to get hot.