Strange Behavior

I'm building a user-editable form and I've noticed that when values are added, some fields don't update until another field is altered. It doesn't matter what filed. It's as if they're calculating one step behind. 

For instance, if I have a field 'output', which displays the sum of input1 and input2. 

With nothing filled out, output displays '0'.

I enter 3 to input1, but output still displays '0'.

I enter 5 to input2, and output displays '3'.

I delete the value in input2, and output displays '8'.

It just keeps going round and round, one calculation behind.

This is just a simplified example to illustrate the behavior. My actual form is using javascript to handle the calculations.

I can't seem to figure out why it's doing this. Is there some limit on the number of fields on a form, before it starts to act funny?

Anyway, much appreciation for any direction.