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Yesterday my 25GH/s Miner from Butterfly that mined at about 26,5 GH/s went down to about 21 GH/s, now i saw with bfgminer that 2 chips aren't working anymore. Any Hints on that?

Bye
Michael

 [2014-02-01 14:48:34] BFL0 51.0C | 5s:21.50 avg:17.49 u:18.56 Gh/s | A:13 R:0+0(none) HW:2/.34%
 [2014-02-01 14:48:34] BFL0a 51.0C | 5s: 3.32 avg: 2.70 u: 2.86 Gh/s | A:1 R:0+0(none) HW:0/none
 [2014-02-01 14:48:34] BFL0b 51.0C | 5s: 3.60 avg: 3.04 u: 3.26 Gh/s | A:1 R:0+0(none) HW:0/none
 [2014-02-01 14:48:34] BFL0c 51.0C | 5s: 3.48 avg: 2.85 u: 3.07 Gh/s | A:5 R:0+0(none) HW:1/1.0%
 [2014-02-01 14:48:34] BFL0d 51.0C | 5s: 3.73 avg: 3.04 u: 3.39 Gh/s | A:2 R:0+0(none) HW:1/.93%
 [2014-02-01 14:48:34] BFL0e 51.0C | 5s:  0.0 avg:  0.0 u:  0.0  h/s | A:0 R:0+0(none) HW:0/none
 [2014-02-01 14:48:34] BFL0f 51.0C | 5s:  0.0 avg:  0.0 u:  0.0  h/s | A:0 R:0+0(none) HW:0/none
 [2014-02-01 14:48:34] BFL0g 51.0C | 5s: 3.68 avg: 3.01 u: 2.79 Gh/s | A:2 R:0+0(none) HW:0/none
 [2014-02-01 14:48:34] BFL0h 51.0C | 5s: 3.70 avg: 2.98 u: 3.32 Gh/s | A:2 R:0+0(none) HW:0/none

Did you recently cycle power on it?  I know a few people who have miners that slow down when that happens.  Easiest thing to do in my opinion is to unplug it and set it in your fridge or outside(if you are in a cold climate) for 5-10 minutes then power it back up and see if your chips come back.

also have 50gh unit hashing 46gh/s and 2 chips not working.
didn't try that fridge trick.

I know a person that had the same problem and he put the miner 10min outside and now it's working again :P

Chips probably need a reflow.
When you go from a hot to cold cycle the lead free solder can crack.
Just like the XBox 360 red ring of death.

Quality engineering. Stick it in a fridge.

Like you know anything about engineering.  Since I take the time to READ and not BITCH about things, I know there seems to be a batch of chips that are more sensitive to heat issues and that by cooling them it brings back the full productivity of the units.  Before you go off half-cocked, since I know that's how you normally operate in life, it's not the ASIC chips that have this problem, it's either in the MOSFETs of the ADP chips as several people have added heatsinks to them and stopped this problem.  Since the cases are fairly complex to take apart and put back together, most people would find it easier to cool the unit off and then re-add power.

@Gator: True, but I wasn't advocating people 'throw it in a snow bank' to try to bring the temp down immediately.  The chips in question do cool fairly quickly, but not at room temp.  I set my friends unit in the window while it was still plugged in but not mining so it could pull 30-40 degree air through it before I powered it down and that seemed to work well.  From testing I have done, it can take 1/2 hour or longer at room temp for them to cool enough to get your speed back.