2000 Pontiac Bonneville RPM drops

I have a pontiac bonneville and have had it for a few months now. I have noticed that at any given time that the rpm gauge drops and at the same time the car jerks. The car keeps on driving normally shifting but if I try to accelerate it makes a grinding noise. I have to turn off the car then turn it back on to reset it sometimes it works fine without it doing it again but recently it can do it three to four times a day. Also there were two times that the check engine light turns on I went to get the codes read and it said it was the crankshaft position sensor, went to a mechanic and told me it wasnt that they would have to run tests on it which cost a couple of hundred bucks. PLEASE HELP!