2001 GMC Safari sometimes it wont start

I have a 2001 safari cargo van a major tune-up was done about a week ago basicly just about every censor was replaced for the emmision test and it passed the engine ligth came on after the test it detected the catalictic converter witch was replaced but it is detecting it again why if its new? also there is an idle problem that my mechanic does not know wath it is becouse it only happens to me every time i take it to him everything is ok say i bring it home from work and i drive for about 1 hour go home and 2 hours later i get in the van crunck the engine let go of the key and it dies turn the key again and it dies the only time it olds runing is if i step on the gas and old my foot on it the minute i let go of the gas it dies again so i forget about it untill the next day i start the engine and now everything is ok until the next time what could this be ?
could you please help me thank you so much.