I have a 1990 240 sedan. Last year it started stalling. I noticed this when coming up to a stop sign or turning into a parking lot. I also noticed that while cruising at highway speeds it would loose power for an instant like someone turned the key off and then run normally. It has plenty of power and accelerates fine. It also starts and Idles fine. It just has this problem where the power cuts for a split second and then runs fine. It will do this a couple times in a row on the highway and then not again for several days. At low speeds this stalls the car, at high speeds it just causes a surge.

I've replaced the O2 sensor, air mass meter and the crank position sensor as I have spare parts laying around. The error codes pointed to the O2 sensor but it could be anything. The ECU is a 280-561 and I believe is original. I've heard 561's can do stuff like this when they are old but have no experience to confirm it. I've cleared the check engine light and it has only surged once this week. Other than this annoying problem the car runs great.

Has anyone seen this behavior? Does anyone know how to isolate it?

 

A couple of things to check: The 25 amp fuse and holder coming off the battery.
The fuel pressure regulator, remove the vacuum line line and check for fuel at the vacuum port. Additionally, check all fuses for corrosion.

Good Luck