Fsbuild-2 Flight Planner

I've had the ISG gauges for years and they're perfect! One thin is, I never could get the VNAV to work properly and I'm not sure why. If I want to cross a specific intersection at a speed and altitude that I entered into the FMC, the VNAV function will decend the plan at a steeper rate and the speed is changed to something other than I entered. Is there anyone who could tell me their expirience with how they use VNAV or where I can learn more about it? I've read the manuals and did not find much in there about how it is used. Thank you.