deleting words causes has words to rebuild in text

[Ammo]

As I type in text (or paste text) then hold the backspace button down, it gets to a certain point of deleting then just starts rebuilding a word and I can't go any further back.

For example: I've deleted half a sentence. (say I delete this one), it builds the following :
I've delsentence

How do I stop this?! It gets half way through deleted and adds sentence.

Help!