Lost power all of a sudden. 273 commando.

If you think your timing isn't working correctly, but your vacuum is still working, check your advance weights/springs for proper freedom of movement. Vacuum advance is essentially inop during heavy acceleration and you said it runs fine when it's up to speed (when the load is down and vacuum is up)

EDIT: try disconnecting your vacuum advance and check your timing that way...it will isolate the mech advance for troubleshooting.