Error Code P0340 engine stalls and dies

I have a Mercury Monterey 2005 just lately it started to have a problem at first it would sputter then run fine and only about once a month. Then it started getting worse with it stalling and then it would die after a loud belt squealing sound. It finely had the check engine light come on so I read the code P0340. I had read that this model had a problem with the ECU having water intrusion problems so I ordered a new ECU and installed it myself. But the problem came right back with the Squealing belt sound and the engine just shutting off after resetting the key to off and restarting it works for a few minutes then dies again with the RPM's around 2000.
It has been real bad the last couple of days since we have had heavy rains so that led me to believe it was the ECU. But after replacement still the same. I replaced the Camshaft Position Sensor also and still the same. I have read that it could also be the Alternator and that kind of makes sense since the belt squeals every time it happens. PLEASE HELP!