Fuel problem?

I have a 2007 freightliner century with a 515 DET and I'm pretty sure I have a fuel problem but not sure which direction to start at. It has the see thru plastic fuel filter set up. Sometimes it only has about two inches of fuel in it and other times it almost full to the top. I changed the filter last week because it was almost full to the top (thinking the filter was bad) which I'm sure it was but after changing it I lost power. I messed with it today day, took it out and reinstalled it.. It was about a 1/4 of the way full but now is over half full. I had fuel to pour in it but it was very hard to start and once it did start it didn't want to stay running? A couple of weeks ago I started it up getting ready for my run it was fine until I hit the main road which I pushed the pedal and got nothing but a after few seconds it took off. I thought I was out of fuel but it did it again the next morning which I was almost full of fuel. Some guy's tell me they changed to the spin on type filter with better results. Just wondering if the pump is bad, flapper somewhere or would going to a spin on filter would be the place to start? It does spit and sputter at any time but real bad under a load.. Anybody ever have this problem with a DET?

Thanks Bill

 

Thanks, I have went over every fitting and line more than once and I can't find a leak anywhere. I took a can of ether and sprayed every line and fitting with it running which would have let me know for sure. I started to buy a fuel pump a few months back but freightliner talked me out of it saying they almost never go bad!! Thanks for the help.

 

Thanks, I have a new one and started to change it today but got lazy and didn't do it. If it runs like crap in the morning I'll put the new one on it.

Thanks Bill