I'm using the newest version of the 757 for FSX SE and I'm still having an issue where whenever the aircraft tries to level off at an altitude below 10,000 ft due to an altitude restriction in the FMS, the aircraft all of a sudden starts to climb and the speed increases to 250-260kts. This happens on every flight now and I would have to resort of using VS mode to continue the descent.

Yes, if your not entering a specific STAR for arrival the FMC will use DEFAULT settings. 
You control the altitude in the FMC. Whatever altitude you enter is what the aircraft will fly.
With that being said,  if you are flying and are going to fly at 8000 ft for example, you need to lower the altitude selector to an altitude lower than the altitude required.

What I do......   If I am at cruise altitude and I come up on T/D, I have to manually change the altitude selector to an altitude that is way lower than the next decent altitude.  If I don't, then the aircraft will not descend.  The same is true if you are required to fly at 8000 ft.  If the altitude selector is set higher than the 8000 then it doesn't matter what the FMC altitude says.

I think that what Caribflyer is saying, is that even with the Altitude Selector set to 8000, once the 757 starts to level of at 8000', it will suddenly start climbing, instead of actually levelling off at 8000' like it should. :-/

Besides that, in Hotfix 1502, this issue has been fixed. ;)