I went for a rather spirited drive with my Z28 last weekend when my automatic transmission stopped downshifting. From a halt, when I gave it some gas, and at about 3000rpms or so, there was a jolt and the transmission dropped into neutral. The "abs no-op" signal came on and I couldn't move. I turned off my car and turned it back on, and it worked fine. After about 10 minutes of conservative driving, I took a slow left turn, and when I gave some gas, the same thing happened, only this time it wouldn't work at all. The only way I got back was to manually shift through the gears. When it is in drive, it won't down shift at all. Could it be the ECU? or do I have some other kind of mechanical problem? Any help would be great, thanks.