Thank you for your reply Mark.
My procedure is more or less the same as yours. I arm VNAV and LNAV prior to take off and then above 1500', press auto pilot.

I did notice that during climb out the VNAV light went off by itself, and then V/S light came on with window showing 3000fpm climb rate, with the speed increasing.
I haven't noticed this before as I've been too busy watching speed tape. However, when I again pushed the VNAV button it lit, and the V/S light went off...and then the Speed behaved as it should. The rest of the flight was satisfactory.

Hopefully the gremlin has gone ;)
Thank you again.
regards
Andy