I have gone through all the posts concerning A/T, but they have not addressed my issue. Here is the scenario (twice now in 1 hour):Go through my pre-flight and checklists. After all data is entered into the FMC, ground gives me pushback and engine start, along with a runway. All V-Speeds are normal, and display on the Nav display. When I get a new runway assigned with my taxi, I go to the DEP/ARR page and select the new departure runway. I clear the "TAKE OFF SPEEDS DELETED" message in the FMC and go to TAKEOFF page. I reset the takeoff speeds and observed they are displayed on the NAV display, again. They are there. When at the hold short and ready to roll I ARM the A/T. Everything looks fine (V SPEEDS displayed, etc). When I take the active I start my roll. When I get to 80% N2 I select TO/GA (the screw head). The V SPEEDS are deleted at that moment (and the NO TAKEOFF SPEEDS message is displayed where the V SPEEDS message was before deletion). A check of the FMC TAKEOFF page confirms there are only dashes in those speeds, and will not fill them in.There is nothing in the manuals that explains this. This only happens when I have to change my takeoff runway.

whoa! that *IS bizzare... i'll try it at home now and see what i get. i never have seen this.mzx

Forgot to add that when that happens I can't select any commanded A/T mode.