Whats up with my gauges?!?

This has happened twice now. My gauges will just go skitzo on me, all the needles(gas,mph,rpm,etc)drop, and my odo just says something like" /``'' "! Both times it has been at night, the first time I had just finished going the two full gears at WOT(my car is an automatic),and the second time in about a week was tonight when I put the car in park to get out I noticed that it did the same as before! The first time it fixed itself, the second time I had to turn the car off. When I restarted it about 15 mins. later, my " overdrive off " light flashed a few times then went off(it is supposed to light up then turn off just once). Anybody have a clue what is happening? Any help is appreciated.later.

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Avalanche 2000 Sentra SE/PP, JWT POP, Advanced Timing @ 17 deg.,205/50ZR16 Bridgestone Potenza RE730s, and coming soon...Stromung midpipe and custom rear section w/Apex i muffler