Wanted to share my expierences, since this is getting anoying:
I am pretty much experiencing this Bug since 2.1 is out.
However a few days prior to June 13th the Problem disappeared.
On June 13th using up to date Addons from that day, I found out that the "GridStatusMissingBuffs"-Module, which I enabled that day for the first time, was causing this Problem for me. I could reproduce Lock-Ups simply by enabling this Module. If I disabled that one everything went fine.
Unfortunately this was only for a short period of time and after updating my Addons a couple of days later the Problem came back - even though i do have "GridStatusMissingBuffs" disabled.
Yesterday I reverted to an ealier PitBull-build (39xxx), but didnt have the Opportunity to test if the Lock-Ups are gone again.
Using ATI-GFX btw...Unitframes used are Pitbull and Grid.