Does continuous passing of wind, with a slight trace remaining on the undergarment, nullify Wudu' and constitute Najasa (impurity) that prevents the validity of prayer?

If the emission of gas (flatulence) is continuous, such that it does not cease for a time sufficient for ablution (wudu) and prayer (salat), then it is obligatory to perform ablution for each prayer after its time has begun. With this ablution, one may pray as much as one wishes within that prayer's time. However, if it does cease, then one should endeavor to find the time of cessation to perform ablution and pray with a valid state of ritual purity. As for the trace of feces on clothing, it is obligatory to purify oneself from it if its presence is confirmed, and not merely a suspicion. One should beware of excessive stringency and over-investigation in matters of purity.