So I´m back with the results with updating the libdrm since it's one of my suspects.
So far I've build & installed the latest version o f librm (Release Q1 2011). The system is much more stable after that. I´ve only hat one kworker spike for about 5 Seconds since, which in my case is an improvement.

Here is what i did:

1.) Download & unpack the latest *stable* libdrm release:

http://dri.freedesktop.org/libdrm/libdrm-2.4.25.tar.bz2

2.) Install libraries / headers required to compile libdrm

sudo apt-get install libpthread-stubs0-dev

3.) in a terminal within the unpacked libdrm directory, do
./configure
make
sudo make install

4.) reboot.

Luckily I was able to capture that one last spike, too. Again, the intel driver, however a different call than before.

# Events: 8K cycles
#
# Overhead          Command                    Shared Object                                       Symbol
# ........  ...............  ...............................  ...........................................
#
    26.11%      kworker/0:1  [i915]                           [k] i915_gem_execbuffer_relocate_slow
                |
                --- i915_gem_execbuffer_relocate_slow
                   |          
                   |--99.79%-- sclhi
                   |          |          
                   |          |--84.92%-- i2c_bit_add_numbered_bus
                   |          |          i2c_bit_add_numbered_bus
                   |          |          i2c_bit_add_numbered_bus
                   |          |          i2c_bit_add_numbered_bus
                   |          |          i915_gem_execbuffer_relocate_slow
                   |          |          i915_gem_execbuffer_relocate_slow
                   |          |          i2c_transfer
                   |          |          drm_class_suspend
                   |          |          drm_class_suspend
                   |          |          i915_gem_execbuffer_relocate_slow
                   |          |          drm_fb_helper_initial_config
                   |          |          process_one_work
                   |          |          |          
                   |          |          |--78.40%-- worker_thread
                   |          |          |          kthread
                   |          |          |          kernel_thread_helper
                   |          |          |          
                   |          |           --21.60%-- process_scheduled_works
                   |          |                     worker_thread
                   |          |                     kthread
                   |          |                     kernel_thread_helper
                   |          |          
                   |           --15.08%-- i2c_stop
                   |                     i2c_bit_add_numbered_bus
                   |                     |          
                   |                     |--83.99%-- i2c_bit_add_numbered_bus
                   |                     |          i2c_bit_add_numbered_bus
                   |                     |          i915_gem_execbuffer_relocate_slow
                   |                     |          i915_gem_execbuffer_relocate_slow
                   |                     |          i2c_transfer
                   |                     |          drm_class_suspend
                   |                     |          drm_class_suspend
                   |                     |          i915_gem_execbuffer_relocate_slow
                   |                     |          drm_fb_helper_initial_config
                   |                     |          process_one_work
                   |                     |          |          
                   |                     |          |--79.71%-- worker_thread
  .....