I am experiencing the exact same problem as the original poster!
Premiere CC 8.0.1 (21)
I am editing with a combination of P2 footage and DNxHD 290 footage. (I originally had some AVCHD footage, but then I transcoded all of it to DNxHD 290 -- but this did NOT help.)
During timeline playback, my CPU utilization never goes above 15% and at least 12 GB of RAM is staying free, so I am pretty confident this issue is NOT caused by lack of computer resources.
System specs:
Windows 7 64 bit, SP1
RAM: 24 GB
CPU: Intel Xeon X5680 @ 3.33Ghz (12 threads)
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