hash rate has gone up 400gh/s since i checked this morning, probably why, mine has gone down compared to earlier blocks today.
plus more variance on smaller miners.

Actually PSU's are most efficient when they are at about 50% load, not 80%. Just sayin

however all PSU efficiency diminish as the caps get old.
I just felt like making a Generalized factual comment haha... MORE FANS!
here is another all PSU's that go up in smoke tend to have lower efficiencies

When I tried the installer for 12.8 for my 7790, it said there was no compatible hardware found but going through the device manager and pointing to the folder where the installer extracted, it installed successfully and has been running stable since. When I had the latest driver installed, I had about 275 mh/s for bitcoin and 33 kh/s for litecoin. After going to 12.8, I get 308 mh/s and 220 kh/s.

I'm getting 245/440/456 Mh on a 3x 7870 rig running 12.8 (using MSI for safe temp overclocks).

Right! Thank you! I've corrected my original post.

Here's an good overview about psu efficiency/80 PLUS efficiency levels:
http://en.wikipedia.org/wiki/80_Plus

So did I... :-)

Have you tried to just lower the Intensity (e.g. switch it to "d") for a moment to be able to use the reader?

Something wrong with the following blocks:

18528 - my reward at 60%
18526 - my reward at 0.03%

Why does Slush's scoring system punish heavily for adding another worker?

See below how my reward was wayy smaller from block #18526 after adding mighty 600 MH worker at 2013-06-12 07:07. My other 200 MH worker reward from block #18525 was 0.096% of next block's reward which had 200 MH worker running non-stop but 600 MH worker running only the last 12 minutes. Block 18527 had both my workers running.

#   Block found at   Duration   Total shares    Your shares    Your BTC reward    Block #   Block value   Validity
18527    2013-06-12 07:46:05    0:26:42    4758169    287    0.00128216    241071    25.08490896    92 confirmations left
18526    2013-06-12 07:19:23    3:52:04    43322066    780    0.00000042    241068    25.33501327    89 confirmations left
18525    2013-06-12 03:27:19    0:39:17    7372985    117    0.00043630    241039    25.12242555    60 confirmations left

same here:

18526   2013-06-12 07:19:23   3:52:04   43322066   17683   0.00000268   241068   25.33501327    85 confirmaciones pendientes
18525   2013-06-12 03:27:19   0:39:17   7372985   3044   0.00966817   241039   25.12242555    56 confirmaciones pendientes
18524   2013-06-12 02:48:02   4:42:35   53459272   21672   0.01090996   241033   25.03670000    50 confirmaciones pendientes
18523   2013-06-11 22:05:27   1:15:25   14487253   5731   0.01006198   240997   25.52886700    14 confirmaciones pendientes