I've heard many times of the digital clusters failing. Most likely it needs to be rebuilt/refurbished. I can't recall who rebuilds them, but I know I saw the answer long ago on the Riviera Owners Association.
http://www.rivowners.org/If you can get your hands on another, known good one, try swapping them out. It's a pretty common failure over time.
_________________
'98 Buick Riviera; Gen V 3.25 pulley, N*, VS Cam, 180 stat, Custom PCM (Powrtuner), headers & HF cat , 42.5 lb inj.
Grant-built/PRJ/Thrasher trans 3.29 gears, 7/8 chain, zzp hardened IS
