My 98 318ti has been acting up the past 2 days. While driving in 3rd gear around 20mph, the rpm kept bouncing into redline back and forth rapidly, but the engine wasn't actually revving as if it was in redline. It went away later in the day, but it reoccured today. The car also stalled once in the middle of a turn. What could be the problem? Could it be a broken speedomter?