93Camry Problem Help needed

I've 93 Camry LE. I took it to mechanic about four months ago and he changed some wires that connects spark plugs. Since that day, I've been having the following problem. When I hit a bump, all the gages in dash board, speedometer and a/c panel would stop working. I hit another bump and they would start working. I thought a wire may be lose but I realized that when A/C is on, same problem occures without even hitting a bump. Gages generally come back to life in a day or two, sometimes even earlier. I thought A/c may be shorting the circuit, so I removed a/c wire from A/c compressor. Problem should be solved now, but NO. I hit a pothole and here we go again. Nothing works. Interestingly, when gages are not working, after driving 30-40 miles, my O/D warning light starts flashing. Warning light doesn't flash when gages are working. I don't know what the problem is and don't want to take it to a mechanic as they don't know what's going on.