Q: Engine noise, RPM fluctuation, ABS, 4x4 warning on

Several days ago, the dash in my car started showing the check engine light on. The car went into a limp mode. After I restarted it, the car began to shake badly and the engine shut off. I attempted to start it back up but it shut off again. I took it to a mechanic and he opened the hood. The engine was making clicking sounds. I was told that the problem was the timing chain had been skipping. He had the valves and gaskets replaced and there were no engine noises afterwards. However, another problem appeared. The RPMs would jump up and down erratically. The fluctuation of RPMs was occuring at higher speeds as well as during idling.

One time, I was driving at 90 mph, and it just switched to neutral. At that point, the dash was showing the ABS light on, the check engine light on and the 4x4 error. After taking the car to the mechanic again, the error code appeared indicating the turbo cooler. After having the turbo radiator replaced, and the car was restarted, another error code popped up showing the throttle issue. However, the throttle body checks out fine. I was told that I would need a DME replacement due to bad ECU. As I have already spent a lot of money, I'm reluctant to pay even more for something that might not be the cause of the problem. What do you think? Should I get it replaced or is there something else causing this?