Sound like it may be either an eng-out prob., or an O2 capacity one. The Virgin 340's have the smaller CFM on them, and maybe can't maintain terrain clearence if one goes out, or it may be that they can maintain the required alt., but don't have the O2 to be able to stay high enough long enough.
On the NOPAC routes between Anchorage/Vancouver etc, and Japan and further, you will often hear speed jockeying going on. Whoever is in front is entitled to stick to whatever speed they choose. Some of the 340 guys even slow down if the -400 behind them complains too much.
[This message has been edited by jtr (edited 20 October 2000).]