Sounds very similar to my problem. What throttle quadrant are you using? Make sure there are no assigned axes to either mixture or prop pitch. I had this problem on my first three flights before I checked various forums and saw this link. I'm on a flight right now and no problems.
After some experimenting after exporting a flight plan I inputted the From/To fields in the FMS. The flight plan gets populated from that, but it doesn't include the rest of the OFP, like ZFW or fuel.
I have SimBrief set up in the iniManager. It has a SimBrief flight plan export feature. I have no idea where it's exporting to. I see the SimBrief downloader can export an ini flight plan file, but I don't know where to put it or if it'll even work.