There is a matematical limitation in both ways the fuel consumption is calculated, it's called division by zero.
Liters per distance (l / 100 km) hit this problem when a car is standing still with its engine running. Traveling a 100 km distance at zero speed would in theory require infinite amount of fuel. When this happens, ECMeter displays "--.- l/100km".
MPG is inverted to litres per distance, so this problem occures when a car is moving and not using any fuel. I felt that "999.99 MPG" better reflects what's actually happening than "--- MPG".
You see the readout changing to 999 almost immediataly becase ECMeter gets data at much higher sample rate than the onboard computer in a car does. You can emualate this "delayed and sluggish" behavior by increasing the UpdateSpeed value in ecmeter.cfg
@Greame43: Can you be more specific about the issues you have? ECMeter went through a rather big update to take advantage of improved OutGauge available in 0.6B. If you have a pre-0.05 version of ECMeter, does that one work? It's possible there are few lurking bugs I haven't discovered...