This is one area you can control. Now back to your team. There can be many reasons for this type of roller coaster performance. You might have a team that plays highly motivated when things are going well. Their performance is based on confidence due to immediate results.
Extrinsic success is like a candy bar.
It gives you temporary highs but not too long afterwards comes the sugar drop. I would sit with my Team or at least the leaders of the team and see if you can't identify some of those trigger mechanism. Talent doesn't always determine this as exhibited by this years Los Angeles Lakers. I realize this answer is a little big vague, however without more specific detail it is difficult to pinpoint any specific cause. I hope these ideas will stimulate your thinking to approach the motivational problems from different angle.