I asked nerdralph if he has an idea and he told me timing straps are interdependent and therefore you have to runtime-mod them in a certain sequence for the GPU not to crash...
I was wondering if you could help me out with my problem. Would be really happy as I've wasted days of time now...
It takes some time, but you can find most of the info in this and other threads on bitcointalk. For example, you have to be very careful changing CAS timing, since one part of the strap contains the mode register settings that program the actual GDDR chips, and another part of the strap programs the memory controller CAS timings. Wolf noted that you can usually get away with the two timings being one clock cycle off, but any more than that and you are pretty much guaranteed to get a crash.