5970 core randomly decides it doesn't like 150mhz memory clock?

[MiningBuddy]

I've got a 5970 that has happily been mining away at 800/150 for about 3 weeks now and for some reason the past few days 1 or the other core has been hard locking up.
I've tried lowering the core clocks back to 725 and even down to 500 while leaving the memory at 150, the bloody thing was still hard locking up on me no matter what. I checked power supply as I am running 3 cards on 1kw psu everything seemed ok, even ran a memtest for 24 hours just to be 100% sure.

Out of curiosity I tried bumping the memory clock up to 600mhz and it has been mining happily now for 48 hours at 800/600 albeit loosing 20mhs, but its stable...

Anyone have any ideas why it would do that and if I should be worried that it has caused some kind of damage to my card or the others by leaving them at 150?  

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[dmcurser]

hmm a little more info on the card like how old the card is ,temps, vram temps

[jjiimm_64]

my suggestion.  run them at 800/300.  you will get about 365 out of each core.

[JWU42]

Jim has convinced me to run at 300 - box is dead stable today - albeit with a tad more power consumption, heat and lowered hashes...

[ssateneth]

Jim has convinced me to run at 300 - box is dead stable today - albeit with a tad more power consumption, heat and lowered hashes...

you say you're getting lower hashes with 300 ram. that sounds like you were using worksize=128. use 256, its faster than 128 for the 300-380 ram range.

[JWU42]

will tweak that in cgminer and advise...

Right you are - a few MH/s bump!

[MiningBuddy]

hmm a little more info on the card like how old the card is ,temps, vram temps

Temps are all under 50c, no idea on vram temps as I'm using linux and can't find a reliable enough tool to check, but given the low core temps I am assuming everything is ok in that regard.
The cards are 3 weeks old.

Managed to get it stable at 800/300, loosing just under 4mhs compared to 800/150 which I can more then live with. Just strange how only 1 of these cards is acting the way it does when I have multiple of the same doing fine with the other.