Rough Idle and loss of power

[04SSHD]

Hey guys, I purchased a high mileage 96 Powerstroke F-250 today. I got it home and noticed that the idle is very rough, rough enough that it is shaking the whole truck when it is in gear. There also seems to be a lack of power while starting to accelerate. I have driven many other powerstroke trucks before and they didn't feel like this one. I am not sure if it is the mileage that would cause this, I have 315k on mine. I also got 13 mpg on one tank, and I ocassionally get a water in fuel light comming on while making right turns. I am new to the whole diesel truck scene so any info that points me in the right direction would be greatly appreciated.

[04SSHD]

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