1993 Lexus SC 400 Rough idle, won't go over 15 mph.

It would help if you tell us what custom work was done. In the mean time, check the catalytic converter, if its broken up insde, the chunks can move down the pipe and get stuck in the exhaust system. This will increase the backpressure and not allow the motor to breath. You can check it by rapping on the converter, if you hear chunks rattling in there, this is where to start. Use a vacuum gauge teed into manifold vacuum, raise your idle to 2000 rpm or so, watch the gauge, if it starts to drop, the exhaust is clogged.

I had coilovers, control arms, and new wheels installed. One thing I did notice was that one side of the exhaust pipes was outputting more than the other. I ended up driving to the nearest Toyota stealership and I started getting that burning plastic smell again, it was smoking by the time I got there. I think it could be the exhaust.

So Toyota will investigate this for you? Let me know what they find.

For the past year i have had similar problems randomly. it started out as a random rough idle sometimes at stop lights, then it started occuring more often aftercoming to a complete stop. Each time it happened, the car wouldnt go over 10-15 mph, and when flooring the gas pedal the engine did not over-rev really load or anything, it was like the gas pedal had no effect on the engine. Eventually the problem turned into the car idleing rough when restarting the car after driveing it (for instance, go into a store, get back in the car and start with rough idle). this eventually lead to the car dying 10-12 times everytime i tried to start it hot. After many expensive parts (vacuum hoses, intake pipe, ignition coils, new plugs and wires, etc), the solution i found three weeks ago ended up being the Fuel Pump ECU. I got a used (but certified) fuel pump ecu off of ebay for ~$90. the install is really simple, its behind the armrest panel on the driverside back seat. I cannot explain the burning plastic smell, i had that too, but that has not returned either. i had a lot of people suggest parts, and i tried most of them, but i really think it might be the Fuel Pump ECU. I also took it to the dealership, they said it ran fine for them and there was no problem. I would suggest that if the problem continues after you get it back from the dealership, try the fuel pump ecu before you start buying O2 sensors and ignition coils, etc. Hopefully this is your solution too.
Good luck!