Auto Throttle/Speed Hold Problems

I am having a weird problem with the autopilot "auto throttle/speed hold" on my KBT F/A-18 Super Hornet. Here is the situation:When I set the speed hold with AP "on" at 250 kts (or whatever speed) the speed never stabilizes. I notice that there is a deviation of +10 to -12kts. I do not however have such a deviation above FL380. It seems to affect the A/T below 10,000. The throttle is constantly adjusting the speed and will not hold the dialed-in speed. I have tested other aircraft and the speed hold works fine. But I also have Abacus F/A-18 and a couple of others and it seems to do the same for all of them.I have asked others who flies the same aircraft and does not have this problem.I have set the weather to calm, clear and no wind. I have reset the fs9.cfg, deleted the STATE.cfg file and nothing works. Does anyone have an idea what might be causing this? Thank you