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Another revival of my thread.First of all I would like to thank all of you for suggestions and great support!!!I didn't do much with LS400 1990 of mine until recently. I guess I needed a Lexus detox after being unable to find a problem. First thing I did was a SMOKE TEST:I performed a smoke test with home made machine and it was a success!!! Smoke test with homemade smoke machine - YouTubeSmoke test on LS400 1990 - YouTubeLooks like there are no major air /vacuum leaks - I was afraid that valve cover gaskets and plugs were installed with airleaks but they came out tight! Two little leaks were present at IACV and its air hose which goes to air intake plastic black box.Also some air was coming from EGR-VM vacuum modulator from its cap which may be normal as it vents air or sucks air through the filter - not sure...Based on smoke test I decided to buy used IACV valve.I tested new old IACV with multimeter for resistance - B1 to S1 or S3 and B2 to S2 and S4 which read around 23 Ohms and with 9v battery for function where I observed closing and opening. I installed it. The car started to run normal and smooth. But the rpms in idle are very low now, around 200 to 300 rpms.My theory : IACV was stuck open and delivered air to the engine all the time causing ECU to add FUEL to maintain proper fuel to air ratio... This is why the rpms fluctuated, engine was running rich and car was trying to go faster by itself. There was too much air coming to the engine via stuck open idle control valve... But now IDLE speed is really low: D or P idle is around 200 to 300 rpms... Way too low. Maybe I should adjust TPS?... But I tested TPS and it seems to be adjusted properly. I've read through a lot of posts on this subject but i've hit a wall of sorts. What if IACV is stuck closed now?... After all I installed a new old one . It didn't stall yet but what if it does... It runs quiet. It does not fluctuate anymore it accelerates nice and smooth. It runs smooth. Still needs some testing. I will drive LS400 for a while and see if ECM adjusts its settings and idle. By the way, later I may get another IACV to see if it improves idle rpms, but at this point I'm happy with solving haunting / fluctuating idle!!! What a relief!One day I may take ECM to see condition of capacitors but for now I just want to drive and not push my luck, given that my soldering skills are be that great lolThank you all for your invaluable input. I made friends along the way and learnt something about cars

I tested voltage Vdc between OX1 and negative battery post as well as between OX2 and negative battery post while in P. Below is the YouTube movies and excel sheet graphs I made based on movies. I wrote down the multimeter readings and correlated then with RPMs at subsequent time points. The engine was not very warm as you can see from the temperature dial of the dash. However my problem occurs at those conditions. Observations presented here were made BEFORE changing ECT engine coolant temperature sensor. Vdc OX1-negative terminal and RPMs (in this video I turned the car with cold engine after a few days in the garage so you can see how RPMs are gradually going up until the fluctuating idle begins): Graph: http://www.clublexus.com/forums/attachments/ls-1st-and-2nd-gen-1990-2000/376455d1450937282-help-idle-loops-up-and-down-from-1100rpm-to-1600rpm-every-3-seconds-20151222_oxygensensor1_voltagetestwhilefluctuatingidle_readings.pdf The same was done for OX2: Graph: http://www.clublexus.com/forums/attachments/ls-1st-and-2nd-gen-1990-2000/376456d1450937343-help-idle-loops-up-and-down-from-1100rpm-to-1600rpm-every-3-seconds-20151222_oxygensensor2_voltagetestwhilefluctuatingidle_readings.pdf The values are approximate and sometimes it was difficult to catch all changes so keep it in mind Also I assumed that .0L means 0mV !!! I wonder what's going to happen once I install new sensor. Happy Holidays to all of Lexus Fanatics