BMW Repair: The BMW MPG and the RPM gauges are going wack, bmw mpg, gauge cluster

Question
A used 2-door 1986 BMW 325
manuel transmission
2.5 engine
153,925 mileage
Recently got a oil tune last week.
Yesterday the problem started.  When I start the ignition it is fine, but after I drive the car and make a stop at a light the RPM gauge goes from a 1 to little low and the MPG gauge goes to 0 to a high. At the same time both gauges go from high to low, while the sound of the engine goes high to low. The car can still run, but I am afraid for further damage.  Please help.

Answer
My first guess would be a bad ground.
Your gauges are electronic and therefore a spike in the system over time will destroy the gauge cluster however it will not do damage to the engine or any other component.  Take out the gauge cluster and check  all the grounds in the vacinity of the cluster, clean them and put petroleum jelly on to prevent more rust.  Other than that I'm not sure; early model 325's had a recall on the gauge cluster to have the screw behind the temp and fuel gauge tightened because of erronus readings.  I hope this helps!
-cheers