Macbook type A1181 sluggish after repair. What could have gone wrong?

I had to replace the CPU fan and the inverter cable of this macbook.

I followed the guides at ifixit: Repair guides for MacBook Core 2 Duo

After reassembly the screen and the fan worked properly but many programs are incredibly sluggish now. For example if I move the cursor along the Dock, it stutters. Typing here is like waiting for the text coming out of a telex. Graphics in certain windows is garbled in very few cases. E.g. youtube frames in Opera.

CPU Temperature is usually low, but with load it rises instantly over 80°C while the heat sink stays at 60°C. The sluggishness seems not to be directly coected to the temperature, as this behaviour is also apparent before the CPU temperature rises.

The steps of repair included many mechanical steps and dis- and recoection of cables.

I had one complete system crash (famous "you have to restart your computer ") since then, but I had those earlier once in a while, so I don't consider it significant yet.

When ruing "top -o cpu" the task "WindowServer" eats easily 50% or more of my CPU when I'm typing or performing other tasks.

The mac has a 320 GB SSD (only 9 GB left :( ), 4 GB Ram.

Which possible sources of my problems do you see? And which do you favour?

• Bad coection after reseating of coectors?
• Damage of solder joints due to mechanical stress?
• loss of thermal contact to heat sink due to old grease in combination with mechanical stress?
• Problem unrelated with the repair.

What are the next diagnostic steps you suggest? Are there any things I should try first to ease my problem?