I am having problems with my 2004 250hpdi have taken it to the mechanic and they cannot find the problem. It was running good then all of the sudden it would not start after running about 20 miles. It would idol really rough and if I gave it any gas it all it would die. They changed the plugs in it and it worked until I ran it again 20 miles again the same problem. We did a compression check on all the cylinders all were good checked all the filters good, we also did a fuel check on all the cylinders and all checked out fine. The boat will not go past 1500 rpm's, it is almost like it is runnig in safe mode. If it is put in neutral and given gas it will run fine at 3000 to 4000 rpms but as soon as you put back in idol speed it will either die or run very rough. There are no alarms going off. I am at a complete loss as to what to do now. If you could possibly help I sure would appreciate it.

Thanks