Hello Goran.
If I got you right, your ESXi sometimes loses connection to DAS. According to VMware Compatibility Guide: System Search there are no official compatibility between ESXi and 9750 8i (but ESXi supports 9360 for example). I think all you can do - is to update RAID controller`s firmware and install latest drivers, but unfotunately it seems that v3.26.00.003vm50 driver is the latest version of 9750 driver for VMware ESXi, and it is for 5.0. But you still can try it. Of course you should check the current version of the driver and ensure that you can rollback to it before updating the driver. For example look at http://notes.peterhampson.com/?p=285
There is some discussion about 9750 (it says that 9750 is no longer supported by VMware).
Also there is possibility that your environment is damaged and you can try to replace controller/change PCI slot/smth else.