Transmission Impossible (to shift)
 

Despite the fun title, I have a serious problem on my hands. Its literally impossible for me to shift into first, second, and reverse in my 951. This first happened to me driving home from work today, and at a stop light I tried engaging first gear, and it simply wouldn't slide into place. It took both hands and me leaning into the lever to limp home from work.

I'm no genius with cars, but it appears its either something with the clutch, or the transmission/synchros/shift fork(s). Can someone confirm or help me figure it out? Other threads suggested a shift lever might be the issue, but I'm not convinced its something that easy (though I'm really hoping it is).

If you can shift with the engine turned off, it's the clutch. If not, it's the trans.

Good/Bad news. The car does shift when the engine is off, but not when the engine is on. Since I now know its something with the clutch, where should I start hunting for broken bits? Master/Slave cylinders, pedal adjustment, or somewhere else?

shift linkage out of alignment?

Look at the shift fork through the inspection hole while someone operates the pedal. If it moves, then you will have no choice but to disassemble the clutch to see what is broken.

Even more news: I just had to back it out of the driveway to park another car in the garage, and it reversed perfectly. I felt the need to take it for a test spin, and everything worked flawlessly. I think this would rule out the clutch plate itself being too worn to engage/disengage gears properly, so what could this have been?

Well, then, it could be that your clutch friction surface is disintegrating, and one of the pieces got stuck between the flywheel and the disk, so it wouldn't release.

Unfortunately, these clutches don't fix themselves.

+1.

You will have intermittant problems until you:
A) Replace the clutch
B) Totally destroy the clutch.

Unfortunately, you are going to replace the clutch really soon.

The only other possibility is the clutch cylinders have air in them. You might be that lucky but I really agree with George....replace the clutch.