Burping and bonking

For the past couple of months I keep experiencing burping, followed by what feels like bonking. I get to around 7-10 k and when I burp, it just empties my muscles. I walk for 15 secs and I'm back to normal speed (when I burp in the first 5k I can run through it).. Then same happens 2 k on. I have tried drinking/not drinking, various energy sources while running, nasal strips to reduce mouth breathing. I'm an experienced runner (3:20 marathon PB) I'm following the same training, nutrition and hydration schedules as I always do. One thing I have noticed, is the closer to lactate threshold I am, the worse it gets (but even backing off by 10-15 secs/km, I'm still getting the burp/bonk). I'm one month off next marathon and am seriously thinking bout not competing. Any help would be greatly appreciated- no suggestion to wild or wacky!