1988 Oldsmobile Delta 88 Enigine back fires and won't start

I have an 88 delta 88 royal brougham. We have had this problem ever since we got this car 4 yrs ago. After you drive around awhile with or without the a/c, and you let it sit for 5-20min it won't start till it cools off a little. It backfires out of the air filter compartment before it will attempt to start. It only happens when it is hot outside.
I was told it is vapor locking, but I thought a fuel injected car couldn't vapor lock. Some one help before I junk it.

Rich