in a desparate try, I disabled the samsung dialer, nothing else, and it seems solved the problem. now, even with memory down to 20MB, program still remains in background.
it seems that besides memory, there are other system resources that programs compete for. so, even if memory is enough, these resources could become limited and forcing program closure.
just my observations, and samsung's dialer does not really offer much more than wm 6.5 native dialer anyway.