i believe that is default behaviour of Reader. when you are on a radio button, you need to use the arrow keys to toggle between them. I believe this is standard behaviour for accessibility.
For the behaviour you want, you will need to fake it.. put 2 check boxes and write code that turns the other off when selected. From the following link Can't I tab through radio buttons? - PDF forms Acrobat | Ask MetaFilter
I think the new behavior in version 7 is the more standard behavior. I do a lot of accessibility work for blind users and the tab to group of radio buttons, then use arrow keys to select the button is what I would expect in a standard application. The reasoning behind this is that if you tab through the radio buttons, the selection on the radio button changes to the current button. For a blind user, tabbing through the form would always leave the last button selected. If you are moving focus to radio buttons and they are not being selected, this is non-standard behavior.
if you want the tabbing behaviour you have stated, then dont use radio buttons. Use check boxes and write code to untoggle the other checkboxes when 1 is selected.
