First, just to double check: are all the units on the same network or different networks? From your description, it sounds like they are in different networks. In any case, because the MAC address on OpenSprinkler is software defined, if you put multiple units on the same network, make sure you change the ‘Device ID’ option to give each device a unique MAC address. Otherwise you will get IP conflicts issues.
Next, regarding the Jumbo Frames, I am not aware of any issue of the EtherCard library (which OpenSprinkler firmware uses) when Jumbo Frames are enabled. From your description, it seems there is strong evidence that there is indeed such an issue. I have to do some research on this. In the meantime, is it possible to disable Jumbo Frames on the three PowerConnect 2808’s and see if this resolves the issue?