There are several possibilities besides those mentioned in the earlier threads.
Have you tried the same setting with another lens?
Possible sticky lens apertures blades.
Is slow shutter speed flash mode on?
There are numerous possibilities...do share a sample pic with the exif intact...
2) Are you being too adventurous, or expecting too much, from the dynamic range of your camera sensor? It's hard to tell what you mean by being either over or under exposed without getting to see your histograms. If by adjusting the exposure/flash compensation you keep getting over or under exposure, you might be working with a dynamic range that is beyond your sensor's capability. I am hazarding a guess that shooting in-doors with flash you can get that very easily.
3) Best way to do things is to use manual settings, manual f-no, manual shutter speed, manual flash output. Once you learn that, you will never want to auto anything after that. First shot to check if your guess of the settings is close enough (you will get a good sense of it with experience) and the second shot with a fine tuning of the setting to nail it. Third shot should get it dead on. Otherwise, do a first shot when there is no action to check the exposure for the scene so when something happens, or which you expect to happen takes place, you are set to go. And with manual settings, it doesn't matter which camera, lens or flash you use, you will get them all to work well together.
ts - why don't you just post a picture as requested instead of going to and forth and let everyone speculate exactly what happened????
post the overexposing photo that u took and put up the spec as well,
then we will know what happen.
if you are doing close-up shot,there a higher changes of overexpose due to the power of the flash is too strong
if i not wrong u had set your flash power to the max 1/1 ,
if that so redue the power of the flash to the min. Eg:1/32
Thank & Best Reg
Ryusei kit lens & 1.8/50mm lens