I have a 97 as well. 50 or 60 take your choice. My take on it is the Series 2 cars are that much faster that the speedo couldn't keep up and probably had a case of whiplash. The HUD being digital is the correct reading to use for accurate speed.
My fix for it not being aggrevating is simply not looking at the dashboard speedo. It'* a poor and simple fix, but I figure the thingy (not a motor) that drives the needle is tired or lower quality..hence it'* lack of ability to keep up properly.
Both HUD and speedo needle are driven off the same wire from the PCM in 97.
00 Regal GS: All kinds of cool stuff.