I am having similar problems with DK28, in both the Truly Epic and Quantum ROMs that I've tried. Wipe x3, Odin to 2.1, GPS lock, install the 2.2 ROM and ext4 and DK28 modem, and it is better.... for a while. Then all over again, with the same problems as above.... slow launcher, GPS drops when you really need it. Never had lost/froze phone calls though. But reinstalling takes a lot of time.
So I tried going back to 2.1 after Quantum. Sucked! DK28 isn't perfect by a long shot - that is obviously why Sprint didn't release it. But apps2SD and interface that doesn't look designed by a high-school computing class make it a nicer experience overall. Just have to reinstall every 2 weeks, lol! At least the devs give us a shot at it while Sprint and Samsung poke around trying to fix Samsung's messed up hardware.
I just wish there was a way to save browser passwords between re-installs, as that is the most painful part of re-installing.