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

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We think we are having tranny issues, and noticed that the HUD scrambles up and down when it is shifting into overdrive.
(1) Answer
(2) Comments
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?
The speedometer reads correct. The check engine light is on due to after market fuel cap.
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?