Electrical Gremlin

I have a 96 Oldsmoblie Cutlass Supreme SL 3.1L and it has a eletrical gremlin that happens once or twice a year only. The car was slightly flooded about 5 years ago, water covered the floor enough to short the ABS computer under the passanger seat out but this was replaced along with the padding under the carpet by insurance. What happens is the car will not start the fuel pump relay located on the passanger side front of the engine bay will not close, you can hear it opening and closing very quickly this might last a hour or a day then it would not do it again for along time. I have changed the relay at the time this was happening no luck new one did the same. Last year I lost all power car would not start changed battery still nothing then all of a sudden car worked no problems. The car quit on me once total power lose when I took of the parking brake (head lights come on) has not done since. For the last year I have noticed when ever use the brakes the headlights would shut off for a milli second now 2 days ago the car would not start no power, jumped car it started and ran but the rear defogger would not shut off pulled fuse but button light stayed on. Drove the car home and when I put the car in reverse lost all power unable to start. I disconected battery and checked with ohm meter battery fine. When I hooked battery back up car would not start and speedo spyked out to 120mph (car not running) and when I shut the key off ther was like a power spyke (door locks would operate loudly, interior lights would flash). I pulled the battery and put it on a battery tender and pulled all the fuse from the cabin fuse pannel I also pulled up the carpet to check the wireing harness on the passenger side to look for possible corrosion looks fine. Battery was out of car alnight, put battery back in and all fuses car starts runs fine but speedo is peged out at 120mph have not drove yet. Car is in awsome shape no rust it has 137K miles on it heck the exhuast is original this is the only issue with I am the second owner I got it in 99 with only 33K miles on it. I figure it is a Electrical issue harness, Computer not sure any ideas, Help would be welcomed. Thanks