I have an 05 dodge 3500 5.9L diesel 6 speed manual. And just wondering why is it when I'm going along at about 2000rpm while driving, and coming up to a stop, I'll engage the clutch and the rpm's will drop to almost a stall. (About 400rpm). Then it'll bounce back up to normal idle (about 800rpm). It idles fine, it does this every time I let off the throttle and engage the clutch

 

Have you checked for codes?

 

Just to be clear when you step on the clutch pedal it does this? Fuel filter was done when?

 

There is nothing for code that come up.
This problem happens whenever I let off the throttle.
The fuel pump crapped out a year and a half ago. So I installed a FASS fuel pump in it. So fuel filter is a year and a half old