Have 3.0 24 valve engine has intermitten rough Idle and during driving at excelleration

Depends on what you mean by rough. Could be a cylinder misfire.They sometimes are not noticed as engine speed increases during driving or under light load as when idling in neutral but at idle under load as when stopped at a light in drive became noticable.
Could also be running rich. The causes of either are too numurous to list.Consult a good mechanic who can actually see and hear the problem to put you on the right track.

check for ghost codes, the light does not need to be on to have codes stored. I would also check your vacuum lines as these can have strange impacts on a idle. check you pcv valve to make sure it is working proper

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.

it sure sounds like you have an EGR problem. This valve controls the flow of unburned exhaust gases and re-routes them too the intake to be re-burned. This valve is usually open at highway or cruise speeds. Not usually open at idle. If this EGR valve is open at idle, you get a rough running for sure just like a mis fire and will smooth out as your speed increases. I would get that checked out and if faulty, replace. Please keep in mind that this part is quite expensive new, but it is electrically controlled. good luck!

I had the same problem with '99 GA I changed the idle control valve , solve the problem

Im not sure if you got it fixed yet! It could possibly be idle air control valve. But here some more suggestions! I had a 94 Safari van 4.3 liter I was working on for about three days and I finally figured out what was wrong with it. It would run fine for a couple minutes and as soon as I hit the gas and let off it would surge, but anyways I ended up changing the distributer, after the fuel pump and filter, mass air flow sensor, egr valve, idle air control valve, map sensor, and ignition module. So that might sove the problem if you were on the same track with it as I was. Hope this helps ya!

there are 2 things that usually cause this, one is a dirty intake idle control valve and the other is hard engine mounts, I would say that both most likely apply to your car. try cleaning the intake with the following procedure from Ford.
Here is the most common cause of surges and stalls and low idle rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.

Your EGR valve shouldn't open up during idle or the engine will run rough and may even stall. Remove it and clean the plunger and port with a wire brush and some carb cleaner. Test the diaphragm with a vacuum pump. If it doesn't work after cleaning, or the diaphragm is bad, buy a new EGR valve.

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.