Always remember that our nightmares chase us with the unfinished business of the soul. So let's pay close attention. Here, you find yourself "ascending," climbing upward, often a metaphor for trying to attain some goal, even a spiritual journey.
I find it odd that you're in third gear. I don't know about where you live, but here in the foothills of the Rockies, we rarely get out of second when we're huffing uphill. I suspect, rather, that "third gear" is a metaphor. The third stage of any journey, for example, is often its completion. In your dream, you do reach the top but, alas and alack, just as you're reaching your goal, your legs telescope into ten pins and you can't push in the clutch. Now, what is the purpose of a clutch? To shift into a higher gear. So here you are, on the very verge of finally getting somewhere, achieving some higher aspiration, but instead you just choke, screech on the brakes, come to a dead halt. Is this a familiar pattern in your life? I bet it is. But never fear: Your dream says there is something pushing you, urging you on, something that won't let you stop.