this sounds a lot like my problems and those were caused by a bad OCV solenoid which is a cheap fix. As I said, have them pull the fault codes and if theres P1349(if I recall it correctly), it's the OCV

 

Had the same problem on my '98 GS400, got it fix 2 weeks ago. Been driving now since and no rough idling. My mechanic said he had to replace the intake manifold gaskets. Hope this helps.

 

I had the same problem..I had to clean the MAF sensor with the cleaner they have at autozone which is like 7 or 8 bucks..but the OCV sensor had messed up because my car is way too low so me hitting bumps oil had got into the sensor so I had to change the valve cover gasket and ended up doing the spark plugs as well and car been riding good since. The OCV sensor on eBay was 90 bucks new from the Lexus dealer

 

I sure am grateful to you guys for your help and advice.

My mechanic took out the Oil Control Valve and cleaned it. The car runs like new which for an 18 year old model is certainly saying something.

He tells me that it did throw a code which indicated it was the OCV but at least I was able to steer him in that direction.

If it occurs again at least we will know the reason and I will buy a new valve.

 

so i did hit the nail on the head... glad to hear your sorting it out. replace the screen when you replace the OCV

 

Both the OCV and the little screen/filter are more or less inexpensive parts so I'd suggest you have them changed before the problem comes back.

Glad to hear it helped though

 

Thanks guys.

George - I will do what you suggest.

 

Does this condition happen hot and cold? Do you stop filling up when the nozzle clicks?it sound like your purge valve is dumping fuel intermittenly this would cause a rough idle or stumble. When you overfill the gas tank eventually it get into the canister assy so when the purge valve operates it send raw gas and not vapor. Try blinding out the hose from the purge valve to the engine and see if the condition occour.