VOR course intercept Ifly 737

I have had a very stable system and been getting good results flying the Ifly 737 with the MCP Combo 2 for some time. I normally take off with autopilot heading selected and switch to lnav at 1000' approx. and everyting works OK. Today I have tried to set up the aircraft to take off and track a VOR course. I dial in the VOR freq. and then set the course with the MCP course knob but the course reading just runs away with itself and cannot be stopped. If I set the course with the mouse on the course knob on the screen everything is OK.Anybody any ideas?

Richard Halton