Like the others said, lose the switch, replace the relay.

$200 for an 89 240 with 87K? Sounds too good to be true. That's almost a junkyard price. Did the car sit for a long time?

Sounds like an air problem.

Cheap things you should probably do anyhow, (if you haven't already done them) before you splurge for an AMM.

Clean the air path from the air intake in the front of the filter box all the way to the throttle body. Add new air filter. Clean the throttle body. $5. Another $20 if the large hose is damaged or collapsed.

Check/clean/replace vacuum lines - broken/clogged ones can cause the symptoms you describe. $10.

Check/ replace rotor & cap. $10. (You can add in a new wire set for $35 if visible arcing at night.)

Clean/replace flame trap. $15, which includes all the replace gaskets, including the intake - removing it makes the flame trap service go a lot easier.

$35-$55 total, $90 if you splurge for the wire set. Add in oil and filter for peace of mind, and for about $100, you've completed most of the routine (and probably neglected) maintenance items for the car.

Somewhere on your to-do list add in a timing belt. $30 with new tensioner. PITA job, labor intensive, but not too difficult.