my 1992 Honda Accord ex will not start after it has sat in the sun.

it started years ago, but only when it was hot and sunny. park it in the shade and it was ok.
now it won't start after sitting in the sun, leaving windows open use to help, but not any
more.  we figure it is a relay switch. but not sure which one. starts great first thing in the
morning.  now it seems to be when it is warm outside.