car runs bad after 20 minutes
 

I own a 40th anniversery. auto, with 60 thousand miles. it has been a wonderful car. this year after i took it out of storage, it ran great for 125 miles and then,
when in drive at about 45 mph it starts to miss baldy. the more i run it the worse the condition gets. it almost feels like vapor lock on the old cars. i took it to a vet "expert-haha" mechanic. he changed plugs and wires. and it made almost no difference. he said might be torqe converter - i took it to transmission specialist and he says not any transmission problems. now i will run it for about 20 minutes and it starts the same thing over again and as I said i gradually gets worse. also when hot and sits a while and then I try to start it it hardly turns over. any Ideas will be greatlly appreciated. I might have to sell because I don't have thousands of dollars to throw at it to figure it out.