Kimmoj,  The issue likely is due to the VMware version. The Dell Custom ESXi version has the drivers specific to the Dell servers, where the VMware version doesn't. So it is far more likely that the Dell version will run smoother and more stable, since the drivers are specifically designed. You can find the 5.5 U2 version here. Let me know if this helps.
Decided to upgrade our ESXi 5.1 to 5.5 u2 on our R610:s to get the new hotness, and did it the same way I've installed all along - just grabbed the latest ISO from Vmware. This has worked very well with both 4.1 and 5.1; the systems have been rock solid for years.  5.5 u2 however has severe iSCSI issues, and the machines just hang, drop the iSCSI connections or both, this actually ate one of my VM's. Thank heavens for backups.  I didn't think about grabbing the custom ISO from here, but does that solve these extremely annoying stability issues? I've tried both using software iSCSI (the way it had to be set up originally with 4.1 to get jumbo frames) and now with the built- in hardware Broadcom NIC's, but the moment you put severe pressure on the storage the iSCSI starts dropping and causing catastrophic issues with the entire cluster.  In the cleanup process I've applied all the latest firmware patches, BIOS is now at 6.4.0, I believe it was.
R610 highly unstable with stock ESXi 5.5u2 - would custom iso help?