Arrow keys (not the scroll lock issue)

I've got a worksheet with 10 unlocked cells in a column, all other cells on the sheet are locked and the sheet is protected. Only the unlocked cells can be selected.

Clicking in the top most cell (row 1) allows me to select that cell.
Pressing TAB or ENTER will move the cursor down one row at a time and from row 10 put me back in row one. That works fine.

However, clicking in the top most cell and pressing the down arrow on the keyboard will move the cursor down one row only 3 times. At row 4 pressing the down arrow will move the cursor to row 3 (up).

Clicking in row 5 and pressing the down arrow will move the cursor to row 6, 7, 8, etc, however pressing the down arrow from row 10 will not move the cursor to row 1 as TAB does, but to row 3.

um.......what have I done here?

any ideas anyone?

It's 25mb.