'78 KZ400C Boggs under heavy throttle

This is my first bike and i'm having a great time learning her ins and outs, trying to keep her on the road.

Problem:
I was near full throttle in 5th gear around 75mph when suddenly i felt a heavy drag, change in exhaust sound, and deceleration. I let off the throttle and felt a slight increase in the drag, and then flicked to full throttle and i felt less drag, but i was still losing speed. I got off the highway and pulled into a lot to have a look. No fuel leaks, i didn't see any steam or bubbling anywhere around the engine or carbs. Exhaust was the same as always at idle. I popped it into first and started off slow, things felt normal up to 15mph, i shifted to 2nd and at half throttle the exhaust sounded like it was bogging and choking, it was a deeper sound and i was not getting any acceleration. I was able to limp the last 5 miles home, never above 45mph in 4th because it just wouldn't push any faster. If i was careful not to push the throttle to hard i could get my speed up but at some point it would bog no matter what. I don't have a tachometer so i don't know what the rpm was at, but it felt like the problem was happening at the same rpm in each gear.

First step is to check the Spark plugs, and post images here so you guys can better weigh in on possible solutions. I already have a set of Carb Rebuild kits on the way, and I'm wondering about this diaphragm that i keep hearing about but haven't seen in any kits.

Has anyone had this problem and what was the solution?

Does the bike have the stock airbox and air filter? Ed

Your description that it seems to idle fine and operate normally at low RPM makes me think it's not getting enough fuel.
Vacuum in tank maybe? Dirty carb bowls? Loose at the carb mounts/vacuum leak?

What does the interior of the gas tank look like? Paint, rust, debris etc.

Clean the tank strainer and recheck.

Pics of plugs?
Steve