hello Mr Kilmer

i have a lexus ls400 1991, for a while now the rpm seems to work in a very odd way, when the engine is cold my rpm is at 1000 on idle, when selecting D, rpm drops to 500 - 600, as the vehicle warms up and reaches operating temp, rpm reaches 1300 - 1500, when selecting D rpm drops to 1000 and jerks the gear real hard, when the car is cold if i switch on the A/C, vacuum pump only works properly on idle but on when in gear it remains on 500 - 600 rpm, but when the car is properly warm and the rpm is high as mentioned before on 1300 - 1500 the RPM goes to 750 when A/C is switched on, 750 has always been the rpm for the A/C compressor but now when the engine is cold the RPM does not work properly at all, i used to suspect my spark plugs and transmission as both of them were worn badly, recently i changed both of them but it didn't work. i do have my suspicion on the air intake system as its extremely dusty over here and my air filter is kind of old.

Please advise. thank you

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