check the condition of the braided ground strap from the head to fire wall and renew its connection to the firewall. sometimes the strap looks fine at the firewall, but will crumble when pulled ot twisted.
try disconnecting the cps wiring harness, clean contact areas with electrical contact cleaner, lube with dialectric grease, and reconnect.
have you checked for spark when it doesnt start? spark should be a blue snapping spark. a weak spark will keep it from starting but it may be enough to start when using starter fluid.
what condition are your battery cables in? my 89 respnded very well when i did a battery cable and grounds upgrade(added another ground strap next to the oem strap), along with a redtop battery. what it did was it enabled the starter to consistantly spin very quickly because there was no voltage loss. getting a constant 12volts to the starter is very important on the renix engine(87-90).
other things i did to get a better start up was a throttle body and IAC cleaning along with a top end/combustion chamber cleaning(i used seafoam through the brakebooster vacumn line). there is also a ground wire in front of the window washer fluid resovoir that i renewed the connection on the drivers side fender wall.
what condition/age are the plugs, plug wires, rotor, and cap in? have you checked that the plugs are still gapped at .035?