Finally it's seems I've got 2 stable A1200's (rev 1D4 and 2B). No more graphic corruptions. I've put an 80mm fan in them which fit nicely even with the lid closed. This is also possible because of the angled CF-IDE adapter I bought from Kipper2K. The 40mm fans I bought from amigakit couldn't handle the job. But in my rev2b A1200 I still need one 40mm fan which I've put underneath the ACA 56mhz card (fan is not shown on the picture).
In my rev 1D4 with ACA 28mhz card the 80mm fan alone is good enough.
This 80mm fan doesn't make a lot of noise.
I don't know if the heatsinks are still necessary, but I decided not to remove them because everything seems to be stable now (running for 3 hours)...
Btw: rev 2b has timing fixes, rev 1d4 has not.