Toyota 2L-TE engine warning light

I have a 1990 Land Cruiser LJ78 with a 2L-TE EFI Turbo. When running, particularly while warming up for the first few miles, the engine warning light comes on intermittently, but not at the same time as the turbo light. When the light is on the engine throttle will rev up for a few seconds, and then come back down, and then rev up again - quite worrying when you are stopped with your foot off the accelerator and the automatic gearbox in drive. Reading the error code gives code 4, which I believe means a problem with the water temperature sensor, or its wiring, or that part of the ECU. I have replaced the temperature sensor, which had indeed corroded through, but this didn't fix the problem. I have put another ECU in from a donor vehicle, but that made no difference. Any ideas?