cdp

I do have a real problem controlling the plane on climb-out. The initial climb-out is fine, speed is controlled on MCP, LNAV engaged and VS set to initially 2000ft, which I then slowly reduce as we get higher and the speed then reduces on the ASI. I do trim the aircraft and slowly increase speed on MCP until about 270knots. Things are fine until about FL160, VS now about 1000ft, when airspeed suddenly decreases until a stall occurs. No input from my side can change anything. This has now happened numerous times. I've gone thru all the Tutorials, but fail to find anything. What am I doing wrong? Any help out there? I use WIN10 with the latest update, have an I7 processor and a Nvidea GTX1080 graphics card. Performance is very smooth, but the behavior of the plane not right.