Intermittent My 2005 toyota corolla, runs smoothly. but, sometimes suddenly the engines idling becomes rough and seems the engine will shut off. please help. thanks a lot

engine light flashes ???? Your best bet , take it to a qaulified repair shop , right now , damage the catalytic converter is happening .
Flashing check engine light CarMD

sounds like you have not connected the wiring correctly, don't think there are any free manuals, you could ask in the toyota owners club.

There re few possibilities that can cause no idle issues.To troubleshoot the issue help links are provided below.Click the link below:---
Idle Problem? http://whatisbyme.blogspot.in/2012/05/2001toyota-corolla-idle-problem.html
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Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html-----------
Stumbling and stalling at idle after engine rebuild? http://schematicsdiagram.blogspot.in/2012/03/stumbling-and-stalling-at-idle-after.html

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Cooling fan Shuts OFF and Idle Drops? http://howtobyme.blogspot.in/2012/05/toyota-celica-cooling-fan-shuts-off-and.html

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Rough Idle and Vibration? http://whatisbyme.blogspot.in/2012/05/2006-ford-f150-rough-idle-and-vibration.html

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Steering Wheel Vibrates at Idle? http://whatisbyme.blogspot.in/2012/05/1993-toyota-corolla-steering-wheel.html

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Runs Fine But Idle Is Low? http://whatisbyme.blogspot.in/2012/05/2004-toyota-highlander-runs-fine-but.html

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Vehicle Will not hold Idle? http://whatisbyme.blogspot.in/2012/05/2003-lexus-es300-vehicle-will-not-hold.html

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Engine Idles Rough and Dies? http://whatisbyme.blogspot.in/2012/05/1994-lexus-es300-engine-idles-rough-and.html

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This will help.
Thanks.

Get the Idle air control valve checked inspected.Also its suggested to get more troubleshooting done.For more troubleshooting,click the help,links below:---Idle Problem? http://whatisbyme.blogspot.in/2012/05/2001toyota-corolla-idle-problem.html
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Runs Fine But Idle Is Low? http://whatisbyme.blogspot.in/2012/05/2004-toyota-highlander-runs-fine-but.html
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Vehicle Will not hold Idle? http://whatisbyme.blogspot.in/2012/05/2003-lexus-es300-vehicle-will-not-hold.html
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Erratic Idle and Intermittent engine stalling? http://technoanswers.blogspot.in/2012/04/1997-toyota-camry-erratic-idle-and.html
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Stumbling and stalling at idle after engine rebuild? http://schematicsdiagram.blogspot.in/2012/03/stumbling-and-stalling-at-idle-after.html
---------Cooling fan Shuts OFF and Idle Drops? http://howtobyme.blogspot.in/2012/05/toyota-celica-cooling-fan-shuts-off-and.html
-------Rough Idle and Vibration? http://whatisbyme.blogspot.in/2012/05/2006-ford-f150-rough-idle-and-vibration.html
--------Steering Wheel Vibrates at Idle? http://whatisbyme.blogspot.in/2012/05/1993-toyota-corolla-steering-wheel.html
--------This will help.Thanks.

Could be 2 things; 1) engine oil level could be low, check level 2) could need a tune-up

Rough idle is a very common problem on 4 cylinders, but I believe the 98 Regency has a 6 cylinder in it, right?

Sometimes the coil packs get iffy, and sometimes the EGR Valve gets a leak in it causing a rough idle.

When the accelerator pedal is depressed, it can change things so that the engine seems to run smoothly.

However, I would look for codes to see if anything intermittent is occurring in the engine that may need attention.

The OBD-I scanner can help to diagnose the problem, if anything is out of specifications on the engine in question.

The owner will have to find a technician that has such a scanner.

Possible weak alternator output or weak battery.

You have a bad Idle Air Control valve. This is a dealer item.

My first guess would be a dirty carb/fuel injector set. Run some fuel cleaner through the tank and see if it gets better. You might have to do it through a couple of tanksful. How long since you've had a tune-up? It might just be that time.

Check the fuel system, chenge fuel filter. clean the fuel tank.