When you say lambda sensor, do you mean the oxygen sensor?
Have you tried checking or adjusting your air/fuel mixture? Also known as lambda/on-off ratio.....
If you have a lower chamber pressure leak (in fuel distributor) then your lamda reading will fluctuate pretty wildly. You need a multimeter with duty cycle function to check this. Sears sells a good one for $30.
You can also check your fuel system pressure and make sure it is within specs. Do you have any warm start problems? Have you replaced your original overvoltage protection relay?
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