Q: My car stalls arbitrarily and with no warning and is getting progressively harder to start.

I bought a used 99 Dodge Intrepid a couple months ago. Soon after, it stalled in the hwy at 65 mph with no warning. The tach simply dropped to zero. Other than the cruise control light coming on, nothing was different. I pulled off with my flashers and it started right back up. The next night, it happened again- took a few minutes to start back up. It has since progressively become harder and harder to start. The other night, it wouldn't start for a few hours. So, the online "research" I've done overwhelmingly points to the camshaft and/or crankshaft position sensors as well as a possible cat converter problem. I got 3 trouble codes from the odometer 1. P1684: battery disconnected within the last 50 starts (which I had definitely not done) 2. P0420- Catalyst system efficiency below threshold (bank1) 3. P0432--Main catalyst system efficiency below threshold (bank2). I keep hearing that I should start with the Camshaft position sensor, as its the easier one to fix

My car has 217000 miles.
My car has an automatic transmission.