Power intermittently reduces from 8000 RPM to 5950 RPM. Always reduced to exact same RPM for a few seconds then back to full power. Any idea what is causing the reduction in power?

Guessing here but could be a battery issue or your alternator. Easy to check those at any parts store will test them for free.
May check the owners manual also for why its doing that.

from your description and what you do , the problem is in the auto control unit
it will be in the system to reduce the rpm to idle when there is no load ( auto mains on off controller switching between genset and mains supply)
by turning the auto operation off you are committing the gen set to full rpms regardless of the power requirement
you have the option of having it fixed at an gen set repair shop or leaving it off and starting the genset only when you have a mains power outage
check with an electrician to ensure that it is possible to run it that way as if the mains power unit is not correct , when you start it and you still have mains power , it will destroy the gen set

Jerry, Drive to a good shop. So many things can cause reduced power light unfortunately. The first step would be to get the truck hooked up to a scanner/computer and have the computer scanned for trouble codes. One thing that you could try is to drain the water from the water separator that is mounted on the frame under drivers front door. If the PCM picks up on water, power is reduced until water is removed/drained. It could be an exhaust sensor that is reading faulty. Unfortunately from here its impossible to tell you exactly what the fault is because there are just so many engine sensor inputs that could cause your problem. Check the attached links, forums,instruction and guides, Good luck
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."

Reduced Engine Power Light Wrench Showing Ford Powerstroke Diesel Forum

08 Reduced Engine Power

Regen Reduced Engine Power Light Ford Truck Enthusiasts Forums

Help reduced engine power light comes on

There may be a choke handle that should reduce the engine RPM. Otherwise contact the Coleman technical support department.

starving for fuel need filter.

I am thinking the switch is at fault, before removing any wires make a diagram for ease in reassembly. you didn't specify what brand or model number but you can get parts from ereplacementparts.com. they have parts for most major brands. Let me know if you need more assistance

had exactly the same problem myself, changed the balance, and the selector until Sugden suggested the relay. this did the job, and my amp is sorted. Shouldv'e changed this in the first place. Good as new. Shaun

what is the exact problem?
when you run it on the mains it uses full power ,when you run on battery it reduces the speed.

halogen headlight taillight/brake light turn signals odo/speedo and horn. Here are the problems… Above about 6000 RPM the ignition will intermittently cut out. It doesn’t do it all the time. The higher the RPM the more it tends to cut out. Reducing RPM brings the lights back to bright. Above about 6000 RPM the lights start to dim intermittently. The higher the RPM the dimmer they get to where the lights almost go out. The engine runs fine if the RPM is reduced. The tachometer records some very high RPMS when the system starts to act up. Max RPM recorded is 19999 which is totally unreasonable. I am looking for some assistance in troubleshooting my electrical system and how to isolate the problem(s).,Start by disconnecting the cables from the generator coils and start up your engine. Problem still there? Do this to find out if your problems stems from the power side or the ignition side. If bike now runs fine, I'd look at the voltage regulator. Have you checked that all your connectors are ok? A grounding problem with a somewhat loose connector would be vibration sensitive, hence the problems at higher rpm. Is your stator plate securely fastened? Is your engine grounded to chassis? There might be a loose fastening in one of you engine mounts. Reply With Quote,,,

The main relay also controls the transmission and will cause an intermittent interruption of power and indicates its going bad. about a $60.00 replacement cost. the relay is located under left side of dash.