bug#14838: 24.3.50; repeating next-line or previous-line is broken

[Stephen Berman]

Starting with revision 113314, I observe the following:

0. emacs -Q
1. C-h n to visit NEWS
2. Type C-n and hold the keys down to repeatedly move point down one line.

At around line 250, the motion gets jerky and then appears to stop, and
the cursor disappears.  After releasing the keys (but with a delay of
maybe a second or longer, perhaps depending on how long they were held
down), the cursor reappears on a line further down (which one seems to
depend on how long the keys were held down).  Subsequently holding down
C-n or C-p makes the cursor vanish and motion stop almost immediately,
until the keys are released.  The same thing happens if, after step 1
above, I type M-> and then hold down C-p (the stop seems to happen a bit
later, maybe after moving up about 300 lines).

With the revision 113360 (this one), the misbehavior begins almost
immediately with C-n, though with C-p it appears to be the same as
above.

When the motion starts getting jerky, my CPU load starts shooting up,
and hits 90%+ when the cursor disappears and motion stops, and stays at
that load until the cursor reappears at the new location.

My processor is an AMD Sempron 3400+ (64bit) with a maximum speed of
1.8GHz.  In addition to the following system information, my desktop
environment is KDE 4.9.5.  My default font with emacs -Q is 
xft:-unknown-DejaVu Sans Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1

In GNU Emacs 24.3.50.12 (x86_64-suse-linux-gnu, GTK+ Version 3.4.4)
 of 2013-07-10 on rosalinde
Bzr revision: 113360 eliz@gnu.org-20130710161817-qw8stt9m23a285vy
Windowing system distributor `The X.Org Foundation', version 11.0.11203000
System Description:     openSUSE 12.2 (x86_64)

Configured using:
 `configure --without-toolkit-scroll-bars CFLAGS=-g3 -O0'

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=local
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t