Not long ago I had an intermittent starting problem with my 88 CRX. After much troubleshooting I found that the main relay (located in dash below driver's AC vent) was failing from overheating. It usually occurred after car had been sitting in hot sun. It would always eventually start. I just didn't realize that opening the hood and sitting for a few minutes while it "cooled"(?) was not why it would start. What was really happening is that opening the doors brought down the cabin temperature and thus cooling the failing relay (easily 100+ degrees). I replaced relay and problem went away.
My friend's 90 Civic SI is now exhibiting the same exact symptoms however, I'm not sure where this relay is. Also I'm assuming they didn't change much circuit-wise from my 88 CRX and still utilize a similar setup.
Before I dig into the books I thought I'd throw this out for any input.
Thanks

 

it's a little black box above the hood release handle. It's held in by a single bolt, 10mm if i remember right

 

Thanks Deetz.
I have found it to be the exact same relay that I use in my CRX. Already got one on order.
Warren