Bad Idle

I have a 1991 silverado ext cab. It has the 305 in it, I have had this idle problem for a couple of months and it is driving me crazy. It started one day like it runs perfect but if you come to a stop and hold the brake for a min. then the idle with slowly go down till it cuts off. It doesnt do it all the time but majority of it, I have checked for vacum leaks and couldnt find any, cleaned the tbi with cleaner. I finally put it in a shop and payed the guy to keep it for 2 days to tell me it needed a new computer, so I wasted the money and bought that and it didnt do anything. I have also replaced the fuel filter and no help. Sometimes it will even do it just idle in park and idle down till it cuts off, and only does it at normal temp. Just need a few more ideas of what it could be. Thanks