I use my variable channels set to fixed and they work fine. If I park too many engines or lighted cars in the sidings, any channel will do this. Even just one of another brand using TMCC/Legacy will cause this.
If the signal is weaker when you connect the variable channel to the fixed channel's track, then the TIU maybe damaged.
I would check track signal strength as mentioned above and the voltage at the rails. Does it matter where the engines are parked? If it does you may have power flow issues.
Have you had derailments that tripped the power? Do you have fuses protecting stuff?
