My 89 Prelude SI, won't start if it's over about 70 degrees outside.

It starts fine if it's cool, like parked in my garage. But if I leave it out in the sun or drive it for a while, no go. Not even turning over or making ANY sounds.  I have replaced 2 relays, 1 under the hood and the main 1 under the dash.  Still won't start when warm. It will always push start OK, but finding a slight hill to always park on isn't always easy. It's my baby.  Had it' since it was new.  Don't want to say goodbye just yet. Help?