I'm almost 100% sure it's not related to which brand of tapes you use.
It is possible that your heads may be "dirty".
I know you should clean your heads when you move from one tape brand to another.
Dirty heads more often will lead to drop frames but you could see "soft" focus too.
I use Sony tapes in my FX1 and it's only out of focus when I fail to focus it so try to clean your heads and test it.
If it's not dirty heads or one bad tape you may have a deeper issue.
The best autofocus in world can be fooled at times and go into "hunt" mode so try to listen to your autofocus motor to see if you can hear it trying to find a focusing point.
It also could be the very easy and simple -there was nothing with strong lines for your autofocus system to lock onto.
Autofocus - Can't live with it - Can live without it!!!