100% CPU load slowing down GIMP

MAIN SYMPTOM
============

I noticed that Gimp was lagging severely at times: While writing a textbox,
there were increasing laggs of up to 10 seconds to write a single letter. When
writing a lot of letters while lagging, I've had Gimp crash. Saving the
document usually makes the lag vanish until I type one or two words, then it
starts to increase again.

SECONDARY SYMPTOM
=================

The other symptom (which might be a hint for the origin of the bug) is that the
cursor is changeing from the "text cursor" to the "move cursor" about 10x per
second (and back!), at times (sometimes the cursor looks normal, though). It doesn't have
to do anything with my keyboard, since unplugging it isn't helping and other
applications don't act funny.

CPU-USAGE
=========
"htop" told me that Gimp was using all of my 4 CPUs to nearly 100%.

GIMP-Errormessages
==================
Starting up Gimp from the shell gives me alot of these error messages:

(gimp:4729): Gdk-WARNING **:
/build/buildd/gtk+2.0-2.24.23/gdk/x11/gdkdrawable-x11.c:942 drawable is not a
native X11 window

(gimp:4729): Gimp-Tools-CRITICAL **: gimp_tool_set_focus_display: assertion
'gimp_tool_control_is_active (tool->control) == FALSE' failed

This is what the shell gives me when Gimp crashes:

(gimp:5561): Gdk-WARNING **:
/build/buildd/gtk+2.0-2.24.23/gdk/x11/gdkdrawable-x11.c:942 drawable is not a
native X11 window
The program 'gimp' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadLength (poly request too large or internal Xlib length erro'.
(Details: serial 7163855 error_code 16 request_code 18 minor_code 0)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the --sync command line
option to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the gdk_x_error() function.)
josh@blackpearl:~$
(script-fu:5756): LibGimpBase-WARNING **: script-fu: gimp_wire_read(): error

Suspected cause
===============
I recently made a dist-upgrade to Trisquel Debian GNU/Linux 7.0 ("Belenos") and
this is a probable cause; however I'm glueless where the problem actually is.
As you see from the version (2.8.14), I've upgraded to the latest version using
an external repository (2.6.10 from the standard repository made no difference, thoug), but the bug persists.
Deleting my .gimp-2.8 directory also did not help. Since it's almost impossible to work with Gimp and I need it
to study, I would highly appreciate any kind of help or bugfix. Let me know if you need any more information!