whenever i start my truck (2002 silverado 1500 5.3 v8, 168,xxx miles) the tach will shoot up to 1,000 then gradually fall (over about 1 minute) to 450-500 and stay there and be very rough because it wants to shut off. although it neither shut off, nor has it given me trouble starting (even on exceptionally cold mornings). i have gone through 2 bottles of crc throttle body and air intake cleaner. i cleaned the throttle boddy really good, and after 2/3 of the first bottle i stopped put it all back together properly, and let it idle. it stayed at about 750 (which i believe is where its supposed to be???) and was perfectly smooth. then i let it be and didnt drive the truck for 2 whole days, as i went out of town. when i got back, i took it out for a drive. i really laid on it and really took off, but a big plume of smoke came out and something felt weird. i pulled into a parking lot, and what do you know, my idle was back down to about 500 even. the next day i used the rest of that bottle on the throttle body, and most of another one (i found a lot more dirt an debris i must of missed the first time), yet, my idle is staying at 500. any info or advice you can give would be much appreciated. thanks in advance.