Looks like my working CPU is batch L426B936. I don't remember the batch# of the one I returned, but it was started with L4 as well.
And I'm using the water cooled, cooler master seidon 120v, though it didn't matter with heatsink I tried to use with the one that was giving me high temp issues. For me the difference was VERY obvious. 100% load was @ 100C on the former and now this one isn't even breaking 80C @ 100% load.