I checked the sensitivity with low noise suspension control of BS and SR3.
Figure 1 shows the result (red: with new control, black:with old control).
Several low frequency peaks are disappeared and noise floor from 30 Hz to 60 Hz was reduced.
Since IFO can be locked for more than 1 hour without any problems, new control seems stable enough for IFO lock.
So, I keep the new control of BS and SR3 from today.