Sounds like gas then no gas mate, have you cleaned out the gas tank its self?
The bike is 40 years old so there is bound to be some crap in there and it will block the gas tap, then let some pass, then block again and your carbs will starve, does this sound like what your bike is doing?
I had a CL175 twin years ago, same thing, crud in tank......lots a luck....Hush.