Center two plugs seem to be loading up?

So I have new plugs in the 1978 kz650. Bike fires up on first kick and it's been running like a champ. Brand new battery, good new fuel etc.  So it's just riding through the neighborhood and after about a half an hour or so ride it starts to cut out a little bit when I get on the throttle. The last time this happened, the two center plugs seem to be fouled. But that was before running seafoam, new fuel etc through it. It's been running fine the last few times I've ridden it on the new plugs. But for some reason I think these center two plugs are fouled again because that's the way it ran the first time has happened. Do you know what might be causing that anyone? Thank you all in advance just trying to get the bugs worked out to run this thing a lot more. 

Your choke plungers might be leaking.  There is a seal at the bottom that wears over time.  zed1015 can replace the seals.

Are the plugs carbon or oil fouled?  Maybe your valve seals are worn?

Another possible issue is weak spark from that coil. A weak spark will gradually foul the plugs. Could be down to points, condenser, poor connection, weak coil etc. 

2 and 3 are fired from the same coil and points/ign pick-up so unless it's a coincidence that is where your issue probably lies.
The choke plungers as already mentioned will also cause similar issues amongst others and regardless of any other faults they will be leaking to some degree so worth looking at as part of any further improvements to the general running and reliability.