SL500 surging at all speeds

[91 SL500]

I need some advise. I have a 91 Sl500 that has been surging at any speed and RPM. The engine RPM falls to about 400 then jumps back to about 1000 in park. I can hear a clicking noise under the engine cover that is in sync with the surging. I took it to the dealer and they said it was the fuel computer causing the problem. The bill for that is going to be $1900.00. So, as you can see, I sure would like to get your input before taking out a second on the house to pay for car repairs.