Your text input method could have been set to Word Method (predictive text) that's why it keeps on changing letters every time you press a button. The Word text input method combines the groups of letters found on each phone key with a fast-access dictionary of words, and recognizes what you want to text as you type. If you want to press a key several times for different characters, you should choose the Alpha input method instead.

Here's how you can change the character input mode:

1 When you see a screen where you can enter text, press Menu key to change the character input mode.

2 Select one of the following options:
- Alpha
- Word
- Symbols
- Numeric
- Text Input Settings
- Insert

Source: Motorola i686 Manual