Speedometer Problem

F355?

As you say, the most common problem is the sensor. If it looks old, I would change it and you'll have one less thing to worry about.

There are plenty of speedo-related message threads on the 348/345 forum which include removal/installation instructions. You may need to create a removal tool (or just cut the plug off the old sensor and use a regular socket). There is also a tutorial on how to test and repair Veglia type 2 sensors, but if you're getting some kind of signal from the sensor (albeit unreliable), then your test equipment might not be sensitive enough to pick up the fault. I guess you could clean the plug on the sensor to see if that helps. Remove the heatshield on the left hand drive shaft for a better view of the sensor and plug (there are two small nuts holding the heatshield on). You may need to remove the rear left wheel and wheel arch liner for better access to the sensor when removing it, but if you have a lift, some say you can access the sensor from underneath.

Yes, it could be an instrument fault if its hunting. Instrument problems are rarer, but not unknown. It's not too difficult to remove the binnacle on the F355 (although the ribbon connectors can be a little sticky... be firm but careful with them). Also lining up the binnacle screws during refitment can be a pain.