Reloading TO situation produces underspeed ...

Guys, I've saved a situation: BA 737-800, lined up and ready for TO EBBREGKK. Everything appears normal on the reloaded FMC/CDU including Cutback at 800 and restore at 3000' - though I need to put flaps 5 back on. The whole flight is fine, EXCEPT the plane doesn't seem to respect FMC Speeds on climb out before flap retraction, and gets right down to stall buffet and stick shaker!! The AT is on, both LNAV and VNAV and CMD A are on - FMC Speed is annunciated, everything should be spot on - but isn't, until we've cleaned up flaps and are back in normal CLB. This underspeeding certainly wasn't a problem when I first reloaded the same flight ... and for sure it didn't come up when I flew the route first time out, before saving it for future re-runs. I also saved a downroute version of the same flight, i.e. just before ToD. What happens now is that the plane reloads miles off-course and too far from the magenta line for LNAV to re-engage without executing a DIRECT TO command via the CDU. Does anyone have any idea why these things might be happening? NGX has been flying and operating just perfectly until now. Many thanks. Best wishes, David