Turn Signal Flasher
With intermittent operation, as described, the culprit is often a poor (cold) solder joint. Since the turn signals and 4-way flasher components are both in the flasher module, that is the likely failure point. Remove the flasher module, open the case, and use a low wattage soldering iron to re-solder each connection to the circuit board. Add flux and solder to make a smooth strong joint and clean the flux after the solder joint has cooled. Test the flasher in the circuit before replacing the module cover, to verify cold and warm operation. Cold solder joints typically are open when cold, but sometimes will close up when warmed up and work properly, until the next cold cycle.
If the problem persists, a replacement electronic relay 5-pin module (EP27) is available for later model vehicles. This module will work with both incandescent and LED lamps (without load resistors). Refer to another article on this forum for replacing and upgrading to the EP27 relay.
The problem may also be a cold solder joint in the ignition switch wiring. A similar procedure to repair a cold solder joint may solve a problem in the ignition switch wiring connector. With the ignition switch open, check for corrosion and oxidation in the moving parts of the switch, since all these components are approaching 30 years old and may just be subject to both electrical and/or mechanical connection failure.