Check engine light flashes at 30 mph, vehicle rocks

Sometimes you can only diagnose a faultwhen it is their / happening ....try earth straps around engine remove , clean and refit

The P0420 is probably caused the misfires. Flashing check engine light is a serious condition that can destroy the catalytic converter if not fixed. Do not keep driving it like this. Get it to a shop to have it fixed.

Take the vehicle to a mechanic for repair.

Check transmission fluid level.

You still have bad ignition coils

Sounds like the transmission is not shifting into a higher gear. 2,500 to 3,000 rpm would be more realistic for a highway cruising speed. The higher rpm may not be a big deal as far as the engine is concerned but it will hit you pocket book pretty good when you loose the mpg that you should get. I would personally either limit your driving until it can be checked or at least slow down so that the RPMs are in the range listed above. Let me know what they find, my curiosity is peaked!

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one of two things ,EGR valve dirty or the idle control stepper motor ,or maybe just a rubber vacumn hose come off caused by deterioration

Is the check engine light on, and if so does it flash repeatedly when problem is ocurring? if so check catalytic converter

Remove the pass side kick panel (Plastic Part) that would be near the right of the passenger's foot. Locate a small blue plug. You jump the terminals together inside the plug with a paper clip. THEN, turn your key ignition on to power up everything but don't engage the starter. Count the amount of times the check engine light flashes. It's like morse code. A solid longer blink indicates the amount of the first number... a quicker flash will indicate the second number. Here's an example: ___ _ _ _ _ That would be code 14. Let me know what code you get and write back... I don't have room to list every code for 90-91 Accord. I'll check back frequently.

As compgeek09 stated, yes the DTC (diagnostic trouble code) should be found out first. And yes, AutoZone or Advance Auto Parts should be able to retrieve the code for you. Then you should be able to tell how severe the problem is.

Yes, you should still be able to drive the vehicle. When the check engine light starts flashing is when you should stop driving the vehicle. Also, if your city/state require smog emissions, the vehicle will probably fail the test. Then you'll be required to repair the problem.

Lastly, Vehicle Stability Control. That light is probably related to the check engine light since it sounds like they both came on at the same time. The VSC controls engine power. Thus, if there is a problem with the engine (check engine light), the VSC can't do it's job.

Once the check engine light problem is repaired, the VSC light should go off as well.