A good first check is to rotate the mechanical advance by hand and let it return and see if it sticks. A timing light can be used to check for spark scatter. Run the engine to about 2700 RPM with a dial back light and see how stable the spark is.

Look for shaft wobble with the advance weights about 80 % out.

Kevin

87 Turbo Intercooled Raider, roller cam, torsen rear diff, LSD front diff, lockup auto with modified converter, V6 brakes, low transfer case gears...