In my case, it depends in which pool is mining:
-Wafflepool: Very stable
-Hashcows: After a few hours stops hashing
-Poolwarz: Same as Hashcows
-WemineLTC: Very stable
Could it be related to the fact that in both with stability issues you can mine and keep different coins?
I have no Idea. I only tried with
Wafflepool, (stable)
Clevermining, (stable but loads of rejects and HW errors (but this is supposed to be normal for clevermining according to the support forum)
Ghash (super stable, this is my testing pool)
I even tried dual (with 10 miners, 20 will crash)
sha cores set to 5 cores
stable for days.
Got 3mhs Scrypt and 35 Ghs BTC