Help me! This one has me confused. I was doing 70mph while on the interstate the other day. My tach. needle, which always reads 2000rpm at 70, suddenly dropped to just below 1000 and hovered around 800-900rpm - at 70mph! When I let off the gas and dropped to 60 the tach went back up to 1700-1800 range and stayed there. Again when I went to 70, the tach read 2000 briefly then again dropped to 800-900rpm. When I took her up to 85 the tach operated normally, reading about 2600-2700 rpm. Whenever I went back around 70 the needle dropped below 1000rpm. I drove it like that for 5 - 6 miles, the whole time my tach reading 800 or so rpm at 70mph. During this time my car drove just fine with no hiccups. I had no problem getting speed and noticed no hesitations when steeping on it. What might cause my tachometer to do this?