This bug has been opened here: https://bugzilla.ubuntu.com/8410

"Often, when opening a message longer than the view pane can display (but
less than two panes long) the bottom of the message is displayed in the
view pane. PageUp will jump to the top of the message, but immediately the
message will jump down to the bottom again (the arrow keys seem to have a
similar behavior). The scrollbar follows this action. The  mouse wheel
works correctly, and using the mouse to move the scrollbar works correctly.

A visual indicator this behavior will occur is a flashing cursor line (from
top of header to bottom of message) at the left side of the view pane. If
it appears on the right, or does not appear at all, PageUp/PageDown work as
described below.

If the left side flashing cursor appears, clicking with the mouse anywhere
in the view pane will move the flashing cursor to that character, allowing
PageUp/PageDown to work within this message, mostly (Additionally, cycling
(F6) back to the view pane will place the cursor between the first and
second letters of the sender's email address). The flashing cursor line
will move up or down with PageUp/PageDown, and as long as the cursor is
only one line high,
PageUp/Down works. If PageUp is pressed while at the top of the message,
the entire header will be flanked by the cursor. On the next PageDown there
will be a 'hiccup' in the movement on the next page down as the cursor
moves to the second line in the header.
 
The behavior is a bit different when the message is more than two panes
long. The message may open at the top or the bottom, and when opening at
the bottom PageUp will jump the message up a page, and then back a few
lines. Repeating PageUp repeats this behavior, and PageDown jumps down and
backs up the same way. Long messages also have the 'flanked header' behavior.
...
With a little more investigation, I can reliably reproduce this bug by
doing the following:

Initial setup: two new messages in a mailbox, both longer than the message
body pane can display.

1.  Start Evolution. Initial focus is on menu bar send/receive mail.
2.  DownArrow to enter the mailbox pane.
3.  F6 to move to message header pane. Search type is focused.
4.  DownArrow to highlight the current message header. 
     (This exhibits another bug, the current message is selected,
     but not visible in the header pane.)
5.  F6 to move to message body pane. (Keep an eye on the cursor. It 
     always starts in the From email address.)
6.  PageDown until the cursor is at the bottom of the message.  
7.  PageUp to the top of the message. Note the one pixel wide
     vertical line flashing next to the header. You may need to hit 
     PageUp an extra time or two to get the line.
8.  Next new message (. or Ctrl-])
9.  The bottom of the message is displayed. PageUp does not work.
...
https://bugzilla.ubuntu.com/attachment.cgi?id=1962
Header with flashing line next to it"