lexus help any ideas

[mirz833]

So i got this new dd and its a 1992 lexus ls400 so i did some work to it like replacing the iacv and some other sensor thats has to do with egr valve bc it was cracked and the reason for these was bc when the engine temp gets to normal the car turns off.So now with these two new sensor on and the engine at the normal temp the rpm needle will drop to like 500rpm or less it will go back up to 1500rpm then in like couple sec it would be at the normal level then a min later it would do the same thing and i goes on and on like that. So when thats happens the car has no power. If i stop at the red light and green turns it will take forever to get to like 25mph i mean forever people have to go around me, but sometimes it would have normal power. So i took it to a mechanic and he did some test and could not find the prob the only thing he said try to find someone that may no more about these cars.The car has no cel on so any ideas? Some people i talked to said ecu throttle body sensors tune up but i dont no.Anyone knows anything about these cars please help me or give me some ideas on what could it be thanks