What would cause the HUD to scramble when the car is shifting into overdrive. on 1999 Pontiac Grand Prix GTP

We think we are having tranny issues, and noticed that the HUD scrambles up and down when it is shifting into overdrive.

by in Red Wing, MN on November 02, 2013
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1 answer 2 comments
ANSWER by , November 02, 2013
Does the actual speedometer read correctly? Have the transmission scanned to check if vss is constant while hud is acting up. Is the check engine light on?
COMMENT by , November 02, 2013
The speedometer reads correct. The check engine light is on due to after market fuel cap.
COMMENT by , November 03, 2013
Have the transmission "scanned" to see if a problem exists! Have ALL modules scanned for any problems. Not talking about a simple code reader but a scanner. The last time you had the codes read it may have been an evap code stored and someone suggested it was the gas cap, but how about now?