I started up Cold and Dark, watched a tutorial and did it correctly, got my FMC all loaded up, and I start flying, I have VNAV engaged and then it starts dropping to Mach .49 and keeps dropping, so I go to speed hold, and the same thing happens, then I manually apply full thrust and its only 85$ percent N1, the speed is going up, but the speed holds arent working, Im using the 757-200 and -200 Freighter, and I bought it two days ago, so its up to date seems to be working now

What I noticed is that the VNAV wont keep the speed once I get above 20000 feet, and then when I click manual speed hold, it wont engage, so I need to manually bring the speed of the Mach I want, and then engage the speed hold, I was doing an event for the FedEx I'm in, and we need to land between 0200z and 0300z at LFPG, and I was going to fast, so I slowed to Mach. 66, and then I was too late, so I sped up to Mach .80, and it wouldnt engage, so I manually sped up to Mach.80 and then set it and it engaged