I installed v9958 on a NMS-8245, everything ok, but when you take 2 minutes turned on the screen flashes black every time faster.
I have a refrigeration unit in the IC.
Checked some documents and some are suggesting to put resistor between pin 4 and gnd / vcc.
V9958 could overheat without it. You can allways try that and see if it helps. Do you have any around > 2K ?