98 Volvo D12 will not rev past 1500 rpm

I'm new to the forum and Volvo's. I have a 1998 Volvo with the D12, it is a farm truck and only used to haul grain short distances. It never has been very powerful, but now it will not rev past 1500 even in neutral. It was running fine this fall, then it started cutting out while driving. It was like the fuel was shut off and the truck would slow down to about 35mph and would only idle, then it would come back again and run fine. I changed the fuel filter and it had a new fuel pump last year. Your help would be appreciated.

 

How do I check that? Can I do that with the computer on the left hand side of the dash?

 

Ok, I tried the test but I am not sure that it will help. Here is what it said for the Diagnostic Messages:

Engine Injector #2
Mech Syst No Respons
Inactive Count: 1

Engine Injector #4
Mech Syst No Respons
Inactive Count: 2

Engine Road Speed
Current Lo or Open C
Inactive Count: 7

Engine Coolant Temp
Data Valid but High
Inactive Count: 7

It also said that there was no oil pressure, but when running the diagnostic test all of the gauges quit working and started working after the test, so I am not sure the computer is working right.

It seems to always have this problem if the engine is hot or cold. You sit there with the truck in neutral and the brake set and push the throttle pedal to the floor and it will not go past 1500rpm. It drives ok down the road, but it still will not rev past 1500. When you are driving down the highway, especially when the engine is under a load like when you are climbing a hill it will cut out and won't rev past 1100 rpm, it is still running but will not rev up. Even if you push in the clutch when this happens, it still won't rev past 1100. After I wait about 30 seconds and the truck slows down to 30mph it will all of the sudden get power and run half way normal. The last time the dealer checked out the truck was about a year ago and adjusted the valves and replaced the fuel pump. I have only put about 2500 miles on it since then. This problem started about 2 months ago.

 

Any other ideas of what might be the problem? When I ran that diagnostic test it didn;t give any codes, maybe I am doing something wrong.