Speedlite 600 EX II-RT inconsistent firing in manual

[Annette1]

I use my Speedlite in manual mode, on camera, for wedding reception candids. I noticed that I will get some great exposures and then about 5 or 6 shots later the image is blown out with no changes to any camera or flash settings and shot from similar distances. Any suggestions?

[MikeSowsun]

It does seem like there is a problem because the EXIF data shows all the same values and it is the farther subjects that are blown out. 

I can't think of any good reason why this would be happening.