1990 244DL

I was getting a check engine light with the codes 232 and occasionally 113. I would disconnect the battery to reset the computer and after about 125 to 150 miles the light would come back. So I bought a meter to test whether I was running lean or rich at idle. Here's what I get.

The car starts up fine. Initially after startup, the meter would read stoich - i.e. fine. Then within ten seconds the readout would drop slowly into the lean range and then off the scale. Results appear to be verified with a mutimeter. I checked for air leaks and can't detect any. I changed the exhaust including o2 sensor, FPR, cleaned trottle body, changed main pump, upgraded the 561 ecu to a 951, did a complete tuneup, etc. I'm just stumped as to what would cause the guage to register off the scale. I mean, a small air leak should make it run lean, but this gauge just eventually drops right off. If I shut the car off and wait a few seconds then turn the key to the II (two) position without firing up the car, the meter slowly returns to stoich. With the multimeter I verify that I am getting.5 volts before startup. The car seems to be running ok, at shakes a bit at idle and once the light comes on it really starts shaking a bit. What could be causing me to run so much in the lean range? Thank you.