First sorry for the length of this post and for the spelling I am dyslexic....
People are giving answers without having the info needed to give a good answer.
The info that Martin needs to give is:
1) How he is trying to do the cut and paste (only with the mouse or using the keyboard ("command+c" for copy and "command+v" for paste) or both or using the menu items?
2) Is highlight the object to C+P the problem?
3) What software is installed and running? (because the problem get's worse over time it sounds like a "memory leak" from a program with a bug in it)
He tells use what he has done and why (looked at answers on the internet) but applying answers you find on the internet without knowing the cause can make matters worse.
Now for a answer based on the information given: As his problem is something that gets worse over time it must be down to something that is happing over time or something that is triggered at certain times it must be software.
Why not hardware? The time frame is weeks not second, minutes or hours then it is unlikely to be hardware.
It is very unlikely to be the OS, Why? because we don't see this problem on all other systems running the same version of the OS X that he is running.
It is very unlikely to be a "permissions" problem because it happens after a time frame of a week. If it was "permissions" then it would happen straight (right away) off and all the time.
So what is left: Installed Software.
Martin says killing the "WindowServer" sort of fixes the problem: Why? All software that uses cut and past uses the "WindowServer" and if any software is overwhelming the "WindowServer's cut&past services" maybe by a memory leak due to a bug then this symptom is to be expected. Also as OS X uses cache systems, both memory and disk killing the buggy process will not clear the cache or fix the symptom. So tracking down the software is not going to be easy or quick.
So knowing the above; even though he kills the Crossover programs and that douse not work we can say that it still might be the programs used with Crossover or Crossover itself that is leading to the problem especially if Martin is cutting and pasting between programs running in Crossover and OS X programs. Without knowing what other programs he uses including what ones he using with Crossover it is hard to move forward.
What can Martin do?
1) Shut the computer down more frequently.
2) Note how long it takes before the first error happens and keep a running log of what programs he has had running in that time.
3) Take note of if the problem starts right after a particular program is used.
4) Find a way of working without Crossover programs find MAC OS X alternatives
5) If you can't move from windows programs and have an intel mac then think about using VMware and bootcamp install of Crossover
6) If you must use Crossover then you need to find the program causing the problem and that is going to take a long time as you say it thats weeks for the problem to start. the points below are for this process.
7) Work with only Mac programs for a while (just over the time it takes for the problem to start and if the problem has not started then the problem must be with programs running under Crossover or Crossover itself.
8) Only work with mac programs and one other program under Crossover if the problem happens the :-) you found it.
9) Only work with mac programs and another program under Crossover if the problem happens the :-) you found it.
10) repeat 8 to 9 until the problem happens :-) you found it.
11) backup your data files
12) re-format/erase your disk
13) reinstall OS X