Your code probably looks something like
   (do ((sublist list (cdr list))
        ..)
       ((endp sublist)
        ..)
     ..)
or maybe
   (do ((index start (+ start 2))
        ..)
       ((= index end)
        ..)
     ..)

The problem is caused by the (cdr list) and the (+ start 2) in the
first line. You're using the original list and start index instead of
the working sublist or index. Change them to (cdr sublist) and 
(+ index 2) and your code should start working.