I suspect the no response from the vss is the issue. I blew up my trans about 3 months ago. After the rebuild, it wouldnt shift into OD at WOT. I also noticed the speedo would jump around at the same point. The trans shop replaced the vss and problem solved. Its been good since, except I blew up the trans again. The trans shop had the line pressure cranked up to much. The rebuilt ot again, bit this time I had them replace the line pressure solenoid with an o OE replacement. None of the second rebuild cost me anything. Between the corvette servos, line pressure turned up, HD master rebuild kit, hardened sunshell, and shift kit, it was just to much for the trans to handle. Without the shift kit and the line pressure being back to OE, its all good now.