Hi,

I usually end up taking off either manually or with EPR and AT armed.   As soon as I engage VNAV though the plane slows, stalls and crashes.   I'm not sure what I'm doing wrong.

Any hints? 

Have you tried switching on the AFCS? Press one of the three CMD buttons on the right side of the MCP, just above the AP Disconnect switch.

When you engage VNAV, the engines will go into CLB mode, which will cause speed to decay if you aren’t pitching down to react to it. Are you maintaining your initial pitch attitude after engaging VNAV or are you decreasing your pitch/angle of attack?