Doesn't sound like interference. Sounds like faulty electronics. A faulty receiver, probably from getting wet, or faulty servos, possibly due to a crash.

First thing to do is find out which item is causing the problem. Easiest way is by substituting known electronics, one at a time. If spare stuff isn't avialable, you can at least try swapping your servos around....so throttle controls steering and vice versa. See what happens? If your steering now works, you know the previous steering servo is bad.

Also try one servo connected to the receiver at a time. A binding servo can cause the battery volage to drop and it seems neither servo responds.

Servos and recievers can be dismantled. Look for green corrosion on circuit boards and clean it off. Look for broken wires, especially broken servo leads.

Although not as common, intermittent items, such as battery holders and ON/OFF switches can cause weird things to happen, so check them out as well. One easy way to eliminate the ON/OFF switch, is to wire the battery directly to the reciever and see if there's an improvement.

There's many other things that could be wrong. You will have to do what troubleshooting you can and if still no-go, come back and post whatever more info you have observed.