Why would he check the clutch master cylinder? If that's gone, and there's no brake fluid, his clutch pedal would drop to the floor with no resistance. That's not a symptom he's described. If you're saying you've had to rev-match before to get it in, that's more than likely a problem with transmission internals, such as your gear synchronizers. Have you ever crunched gears before? It could be a spot that won't engage when you're downshifting due to teeth not locking, and as luck would have it sometimes it's at a point where it will lock on to the lower gear.

I would first change your transmission fluid as requested above, and then secondly, if that still doesn't work, begin looking to either rebuild the transmission (shouldn't have to at 71k but if you or someone else has dogged it...) or replace.

Also, why are you downshifting? IMHO, save on transmission wear and use your brakes to slow the car. That's what they're made for. I would only downshift when you are in a race environment, autocross or road racing.