stops working in the heat

not over heating before it dies. fuel pump, vaccum lines, radiadtor, water pump, relays have all been changed. it restarts usually about 40 minutes after it dies. tuesday was the first hot day and it died off and on all day. we've taken it in to a few different places and no one came up with anything. we heard twice that there is nothing that should be making it do that and once that it was getting vapor locked and on that system it"s impossible. bout ready to take the darn thing out back and shoot it!! company wouldnt like that though. after knowing all that if you can think of anything would be great. thanks- nicki.