Matty...

I can relate to you , because my wife had a similar incident ....suddenly she lost all her energy ..had sweat...dazed eyes...but it lasted for few seconds.That was the worst although she had similar incidents of less intensity before.

That was before she started B12 and Iron. As Rose has pointed out , please do your wife's ferritin test if you have not done so far.

Since then my wife has been taking 2000 mcg/day of methyl B12 and proferrin Iron tablets(Heme Iron). Her improvement graph has taken a curve up since she started her Iron tablets. Although we have not done ferritin test after that.
Study about Celiac and other auto immune disorders ....look for other possibilities !
Do you give your wife glucose etc?when she feels like this ?

(I am no expert .....learning myself)
Good luck...