Problems in Data Connection Mode

I have problems in data connection mode with M1306B. Each modem connected to controller. When I connect such modems to PC, all work perfect. But with controller work only 2 modems from 4. Such problem appear just after 3 month of perfect work of all modems. All settings are equal to each modem.
I tried to set +IFC=0,0 but it not take any effect. May be firmware upgading will help me?
Please, help.

What “problems”, exactly, do you have?

What is this “controller”?
How are they connected?

What do you mean by “perfect”?
How extensively have you tested them on the PC?
Have you been sure to exercise all possible scenarios - especially the error conditions?

Again, what exactly are these “problems”?
How are you sure that the “problems” are in the modems, and not in the “controllers”?

Did you test for 3 months on PCs?

And are all the “controllers” the same?

With connection all is OK, I call using ATD command, then I recieve on both modems CONNECT 9600, and then problems start. ‘Answering’ modem don’t transfer data. “Calling” modem always connected to PC. “Answering” modems (4 modems) connected to Mitsubishi Application Controllers into special GSM-Modem port (RS-232). All controllers are equal, modem settings are equal too. I don’t use handmade RS-232 cables between modem and controller.
So I can’t understand why 2 modems recieve and send data, and remainder 2 modems keep silence.
I tried to connect sniffer between modem and controller and understood that modem keep silence, not controller.
All this devices worked with controllers 3 month without any data loss. But now 2 modems transfer data only when connected to PC.
All this troubles appear not in all 100% of cases. Sometimes I have successful data transfer, but may be only in 5% of all attempts.
I understand that problems are not only in modems (cause they still working good with PC), but I didn’t find any reason why equal modems and equal controllers in some cases work good, and in other - bad.
May be, I must try to use 3-wire connection cable (GND, TX, RX)? But if I set +IFC=0,0 it will not be any difference between 3-wire or 9-wire cable?