Maybe your upstream o2 sensor is on its way out, or shorted? I had a similar issue where my car would break up at WOT (in any weather though). But I monitored HPFP, STFT and AFR. What would happen is the o2 sensor would short out, the PCM thought it saw a super lean condition, so it dumped fuel until STFT was maxed out, causing HPFP to drop significantly.