The service engine soon light came on and as soon as it did my car started to run rough and speedometer stopped working !
Scanned for codes and a code 42 came up saying spark control problem .
Changed Spark control module and coil packs , but problem still there .
Seems like the problem maybe electrical as if I hit a bump things work good for a bit then go back to acting up.
At a loss as where to look now .

I would suspect your crank sensor or associated wiring, especially with the 'bump' issue. The crank sensor is behind the harmonic balancer (crank pulley).

Code 42 is a tough one that is pretty general. Another possibility is the loss of the magnet on the cam sprocket, but I'd set that idea on the back burner with your bump issues.

Would this also cause the speedo to quit functioning at same time?
When the car decides to work properly (for a breif time) everything works ok .... but once the SES light happens the car runs rough and speedo quits ... hit a bump and things work ok until next bump which in the countryside is often.

Sounds like an electrical connection problem/wiring issue.

The Speedometer is driven off the ECM under the dash. It gets it'* signal from a pulse generator on the transaxle and doesn't cross the Ignition circuits except through the ECM under the dash...so the ECM becomes the common point where both problems cross paths.

That was my though, a bad ECU or bad connector. You may try putting the ECU connectors on and off a couple times to see if its a continuity problem first.
Then do the ICM connector.
Happened to me on a damp rainy day.

Could also be a corroded or loose ground connection.

I think the PCM ground is under the ICM mounting bracket on the 89s. I'd check the battery cables & ground bus too.

Another thought...

The wiring harness that carries the Vehicle Speed Sensor and oil pressure sender comes up from the passenger side rear of the motor (behind the power steering pump) to the steel lift ring.

The wiring harness for the ignition module, A/C, and a few other odds and ends crosses over the manifold on the passenger side of the motor and joins up with the harness from the VSS & Oil pressure sender...then the whole bundle jumps the gap from the engine to the body on it'* way to the ECM.

Sooo... that would also be a place to look..harness wires grounding out on the block or some metal bracket on the motor. Those wires were all sheathed in black plastic wrapping, but the car is pushing 20 years old and it'* possible that the protection is breaking down.

I'm betting it'* just corroded contacts in connectors/ground wires, but that it comes and goes with vibration sounds like something shorting out on a part of the motor or body.

Thanks everyone for the help .... got the car running ... replaced the crankshaft sensor , spark control module and ecm ... Car runs like a charm now !

Again Thanks for all the help.