ZOOM CA'BOOM

Can anyone trll me how to fix my "User View" zooming options?
It works fine for a while, and I can move in and out of the object im 
working on with great accuracy. But then, it just decides to make my 
zoom jump in huge steps. Then im either too close, too far or inside 
the object to work on it.
The options on the pull down menu dont seem to help, or change 
anything.
Is there an shortcut key im accidentally hitting to make this happen?
Is there one to fix the problem?
Any advice would be a bonus, thanks.