Strange Current Discharge..

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Have a weird one...

88 Mustang LX 5.0, new battery, new alternator, new cables. Even after changing all of the above, I'll be driving and the battery meter goes from the 3rd mark from the top all the way to the second red mark on the bottom. This is an instantaeous event and does not occur gradually. Usually after 15 minutes or so driving on the battery, the meter pops back to normal. But... occasionally it will run till the battery runs outta juice and the egine quits. This is really driving me nuts. I've tried pulling one fuse a time when it's in this mode but can seem to isolatee a particular circuit. Car is stock and does not have an electric fan... 100% of the time, early in the morning when first started, the meter is normal. Strange event seems tohappen as the engine warms later in the day. Livein moderate climate, San Diego, CA

Help!!!

Thanks