flashing od light

[shandango]

I recently bought my wife a 2000 G20. The od light will flash sometimes when first started and after driving for a while the transmission will surge and the ses light will come on. A snap-on scanner shows code P0500 speed sensor. I replaced the speed sensor and was fine for about 2 days. The car did the same with the same code. A mechanic friend said the battery could also do this, so having an Interstate in it I replaced with a Nissan factory battery. Made no difference. My Infiniti and Nissan dealers are no help. Any advice would be greatly appreciated. Thanks!!