I have a '95 Riviera SC. Keeps throwing P1257 code, but ONLY when car is cold (less than 5 minute warm-up) & under 3000 RPM (2500-2600). If car is warm OR 3000 or above RPM, no code. Happens in the first few minutes after I leave home every day, starting up an incline, boost duty under 100%. Does NOT happen any other time. If I downshift manually to get RPM's up, does not happen. If I drive around the block & do it again, does not happen. Otherwise, runs great! Any ideas? I've replaced everything I can think of pertinent to the problem; MAF, EGR, PCM, EBT, Boost Solenoid & Actuator, H2OS sensor, bad vacuum lines, fuel pressure regulator, injectors, EVAP valve. Thanks.