My car finally refused to start in a hot day so I took out the main relay, find the cracked solder joint & fixed it with a solder iron. My car is starting reliably in a hot day again.
1. Main Relay location in my '91 Coupe. Very easy to find with the kick panel removed on the driver side:
2. This is how the Main Relay looks when it is out:
3. This is how the side looks:
4. Find the cracked solder joint on the circuit board:
5. Zoom in to the cracked solder joint (round crack):
6. I used a solder iron to reflow the solder (with new solder added)
The whole job is quite easy to do if you can solder a wire.
The following are the links to some very informative site about Honda Main Relay repair: