I have a 72 175 that I got going and started having some issues shifting.  It runs really good other than the two instances i've seen below.
It does not do it when i'm just cruising around and keeping revs lower. 

If I get on it and take it up toward red line a few times it's gone into neutral on me.  I don't think i'm accidently clicking the shifter with my foot but maybe I am?
Not sure exactly what gear it was between either but feel like it was in 2nd gear accelerating hard from a stop.

I've also noticed a few times when coming to a stop and am downshifting I can't quite get it down into first.  It's happened where I think i'm in first then try to release the clutch to take off but it's not and it bogs and dies.
Then you can rock it back and forth while trying and eventually it will click down into first.

Only thing i've really messed with on the clutch side is new friction plates and adjusted the clutch cable slack per the manual.

If you come to a stop with the bike in a high gear, often you will need to rock it or roll it to get it to downshift all the way...  that's the normal nature of a dog-type transmission. 

Have you checked to be sure the shift drum retainer and shim are present under the plastic cover on the left side of engine.... forward of sprocket and above pushrod.  You need to remove the side cover to get to it.  Sometimes someone has thrown a chain and the end of drum (under cover) gets broken off.  Shift drum will float and won't shift right or stay in gear.  Just missing the shim can even cause a problem.

There's a spring loaded lever and detent wheel on right side under clutch cover, usually don't give trouble but I have seen spring disconnected or damaged. 

Could be adjustment of linkage, again under clutch cover. 

Possibly dogs are just plain worn out and rounded off...  without disassembly and inspection or at least history of possible abuse it's impossible to know without taking it apart. 

Ha, just started writing exactly what Mark says pretty much. So i'll 'second all that'.
Me thinking check the arm roller spring seeing you were in at the clutch although not directly related as is behind the basket.
Back in the day we replaced arm rollers often as wore out badly as the roller turns--wears on the pin it revolves on then can move it a lot on the pin.
Probably doesn't matter "until it does" as reduces spring tension plus if it was severe the under area of the arm could contact a drum pin.
I'm guessing a bit without looking.
Maybe we had a bad bunch of roller arms one year--not hardened.
Plus check the drum end by the front sprocket.
Also the circlip C clip on the change shaft below the sprocket although if that's gone missing it'll either change gear or you'll have nothing so unlikely that.
Wasn't going to start writing--then i did--just reminiscing from when we sold these to farmers in NZ.

Are you sure it's not just slipping out of gear when accelerating because you didn't get it fully into second? Sometimes you get bad upshifts when wearing sneakers. Downshifting is best done as you approach stopping, then click up to neutral just before you stop. After downshifting to second, while still rolling, I hold the clutch and punch down into first, then up to neutral just before stopping. If not, you need to rock the bike a bit as Mark said.

Thanks guys that gives me some things to check out.
Will possibly take it for a ride this weekend again and see exactly when it’s doing it. Has been too hot to do anything to it

gear lever not bent sideways & digging into the cover.

Haven't had a chance to ride it but took off the side cover and the plastic cover, retainer and shim were there.
Shift lever is not bend or hitting the side cover.

To clarify the shifting issue where it's going into neutral is not when physically shifting gears it does it while accelerating so no input from gear shift at all. I'll be on the gas at higher RPM and it just goes into neutral instantly and since i'm still on the gas it revs to the moon until I can react.

@ Tom P
Right that's what I try to do, downshift coming up to the stop. I don't come to a stop in 4th then try clicking down gears before taking off again.

Problem is when i'm trying to downshift coming to the stop. It doesn't downshift when I thought it had then I go to take off and it's in 2nd or 3rd.  After that happens and it stalls I can then rock it to get it into first or neutral.
 

Starting to sound like a severely worn dog issue how the sliding gear engages with the fixed gear. Or fixed gear has blown a circlip. All a guess without being in there. It's not like those trans to do something like that. There is a pressed on gear on the main shaft but if that moved away it would let 5th gear become less engaged & ramp out. But if the pressed gear moved it'd likely spin from then on which would stop second gear from working--following through.