Mazda 6 engine runs, RPM zeros out

I have a 2004 Mazda 6 with 85,000 miles on it.Friday I went thru the gears getting on the interstate and when I went to 5th the engine reved and the RPM's went to 0, The engine kept running though. The check engine light did come on. When I would let up on the gas the car bucked. Whenever I shifted the RPM's would zero out although the engine was running. I took it to a garage, the oil was low, so we added oil. They thought it might be the Cam Sensor. I took it home over the weekend and went to start it both Saturday and Sunday morning and it would start and run very rough, until it the engine warmed up. I took it around the block a few times and the PRM's still hit zero when I shifted. It is in the garage now and they changed the Cam Sensor and that didnt help. Please help! Thanks