There are two distinct possibilities.
- There is something wrong with the engine / ignition / fuel delivery
- Rider input needs to be modified
In the case of 'something wrong', that could take a while to find out - but tackling a hill in a higher than normal gear at slow speed will tease out any ignition problems; Larry's balancing a coin on edge when the engine is running test will reveal balance issues; Injector cleaner in the fuel for a couple of tanks is the first thing to do for fuel issues.
Rider Input - I don't mean that in a derogatory way. I found the gear lash to be quite significant when I got my first Pan. And it is more noticeable on a heavier bike. My chain driven XS650 which had a sidecar fitted had a similar issue when it took up the slack on the chain when slowing and accelerating. But it's just a case of becoming familiar with what to do to minimise it. Stick a pillion on the back - they will let you know if you are getting it wrong. So will you - you will get bruised kidneys from the thumps that you get every time you bang helmets.
I can make my Pan 'snatch' as you describe. But usually it rides as smooth as silk. Nothing changes between being smooth and being snatchy - except how I am riding.
I do something that the advanced riders frown upon - I like to use engine braking - ie letting the engine slow the bike down. But that means that the wheel has to take up the slack to catch up with the engine. If I release the throttle quickly, particularly in lower gears, I can feel the jerk as the faster turning wheel 'bumps into' the slower turning engine. So I don't release the throttle quickly. First I stop accelerating, then I ease off the throttle a tiny faction more to let the wheel take up the slack. The throttle is still open and keeping nearly the same engine revs. Once the wheel has caught up, I can release the throttle - result - no snatch.
When accelerating away from slowing down there are two areas of slack to take up. One is the slack in thr throttle cable, which I can feel. The other is the slack in the drive train. When I first open the throttle, there is no weight on the engine - so to do that quickly will result in the engine revs 'bumping into' the wheel. So as my slow-down phase is coming to an end, I start the process of slowly turning the throttle so that the slack in the cable is taken up and I can feel the engine beginning to drive the wheel. If going down hill, a touch of rear break will help to keep the engine driving the wheel. That way I am ready to drive away when I get to that point in the manoeuvre.
In short, I never open the throttle to accelerate until I make sure that I have the drive on the back wheel. And I find this to be particularly important if I am approaching a tight bend. I get the speed sorted out first, then make sure I have drive, then I have all the control I need.
It's long winded, explaining it but I do it without thinking about it.
Mechanical things that may help.
The throttle tamer - has an eliptical section - so that more movement at the throttle is required in the first part of the throttle movement.
New cush rubbers in the rear wheel - can make a difference if the old ones are getting a bit loose.
Reduce the amount of free play in the throttle cable. There is a specified amount - but it caters for extremes of temperature. Check the revs don't rise when you turn the bars when it is cold. Make sure there is some free play. And put it back to spec when you have finished practicing !