Crankshaft Sensor

This is my second Aurora and I never had this issue. I was told from a mechanic that I need to replace my crankshaft sensor but I am seeking a second opinion. At times my car will not start, right away. I either have to keep turning the key multiple times or hold the turn until it finally cranks up. When it does crank, sometimes it will run smoothly, at other times it will idle high, between 3000 to 4000 rpms. And it will do that for maybe fifteen to twenty minutes I usually do not have time to wait on it to calm to normal rpms, so I go. It will stay high as I drIve and eventually lower. And it does that throughout the day. At other times, it will crank just fine and stay that way, or as soon as I put it in reverse, it spikes up, put it in drive, it might lessen, drive off, spikes up again, stop it may lessen. Basically anything can trigger it. I am also afraid I am messing up my brakes because if you can imagine it is harder to stop a car if the engine is steadily between 2000 to 3000 rpms, even while stopping at a light. Is that really my crankshaft sensor? Getting so tired of this, need to get it fixed ASAP! Thanks!