Maybe I'm misreading your post but Vnav SPD won't maintain the desired FMC calculated path. It just maintains speed on elevator. Were you originally in VNAV PATH and got outside the VNAV path limits that it reverts to spd and stays there. Or have you opened the spd window? I need a bit more information. What do you mean by the V/S went up to 9999? Is it asking for max ROD? What is the next at altitude/below altitude after the at or above altitudes? Do you have a specific STAR I can refer to. Might get the cogs in motion to visualize your issue.