electrical help

i have a '97 gt convertible. last couple of days i sensed something was wrong with the voltmeter.....this morning got to work, put key back in and all electric was gone ( horn , lights , radio , guages....nothing would come on ) completly dead. lunch time went to check cables , battery and fuses - what ever i could. after a few minutes everything just came on ( wasnt really doing anything except checking battery voltage ) drove car home , had battery and alternator checked ( all good ) cleaned battery connections, still seems like its not quite right. voltmeter seems to dip more when im pulling from the battery, more than usual ( power windows an such )

any suggestions would be greatly appreciated..

one of the first things i did was clean terminals , its been a couple days and no incidents. i will try the cable to the block this weekend. and check other grounds and connections i couldnt get to and see if the power draw is different

ive never had a car go completly dead electricaly then for no apparent reason come back to life. mystery....

thank you for the reply