1996 Toyota Camry High NOX at 15mph, good at 25mph - please

My car failed the CA smog check. The following are the results:

15mph: %CO2 = 14.7; %02 = 0.3; HC (ppm) = 14; %CO = 0.00; NO (ppm) = 1297

25mph: %CO2 = 14.9; %02 = 0.0; HC (ppm) = 8; %CO = 0.02; NO (ppm) = 84

I checked EGR by applying vacuum at idle => stalls car.

What can cause bad NOX at 15mph, but fine at 25 mph? All other measurements are fine. Please help!