hmm.. well, both things happen at low RPM so thats a clue.
At the moment it shifts, the higher gear forces the engine RPM to drop. If the engine is weak at those RPMs and can't cleanly pick up road speed, the transmission/clutch can vibrate while it tries to decide if it should get into high gear or stay in low gear..
Im going to guess that when you let off the throttle any extra fuel in the cylinder/intake/crankcase gets spit out the exhaust and at some point, for a moment, the perfect air/fuel mix is exists for that RPM. So, the engine has power and the bike can take off.
The other thing also involves low RPM. You have a rough idle and it has to be set very high just to keep the bike running.
So, i would think it's a low-RPM / idle problem and could be cured with a carburetor tuneup. Start with a perfectly clean carb, of course. Then get the engine good and hot and then readjust the idle. Make sure your carb's cables are adjusted properly, including the choke linkage.
i think that if you can cure the idle problem the shifting problem will disappear too.