infrequent engine miss

My 99 tourer has developed a miss that occurs maybe once every two or three hundred miles. the symptoms are: traveling at cruising speed 60-80 it appears that one or two cyl are cutting out. engine shudders and power obviously drops. In the past two weeks or so since it has started this all I had to do was open the throttle big time two or three times and it would pick up and be fine. Yesterday I did not increase much throttle to see what it would do. It continued to miss for a couple of miles at which time I opened it up and away it went.

History

Last August I pulled the carbs and replaced the slows for poor idle and adjust the pilots as this bike was not used often . I purchased it last year (6k miles in 9 years) Replaced vacuum hoses intake o-rings plugs petcock diaphragm rear end lube etc. After start up it ran good but did a carb sync and all was well. took it for a run and at the five mile corner it dropped cyl 3 and 4 as far as I could tell by pulling the plugs wires. I thought I might have had a vacuum leak so I plugged off the hoses but it did not change. It would pick up the cyl when the throttle was opened half or more but had a miss at closed or slightly open . About 15 miles or so later it just picked up and ran perfect. I have put on about 7k since then with no issues but now seems to have a similar problem althought the miss is intermitent.

It could be fuel related but it drops the cyl so quickly and picks up like the turn of a switch I am leaning toward a weak spark or similar issue. I will remove all coils and check connections and grounds. I installed new plugs tonight but havent done a run yet.

Give me your expert opinions

thanks

warren