2000 Silverado RPM Fluctuation.
 

My 2k chevy silverado will fluctuate rpms between 35 and 55 miles and hour. It's very annoying. I've read about them and have not figured it out yet. I can turn the tow/haul on, and the truck will not do it. If anyone out there has the answer to it. Please let me know.

lock up torque converter kicking in and out.

I have the same issue with my 2003, V6, C15, 2WD. it does it randomly when starting from 0 from a red light and occasionally between 45 to 60 MPH. I have changed spark plugs, wiring, coil, ignition module, distributor and finally spider injectors, it worked good for the first 3 hours in every repair and then it started again. My case is misfires on Cylinder 5, but I don't know what else to do, it is annoying, and starts to really bother me.

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As said above torque converter bad

Well, believe it or not, I'm gonna kill myself, after all this changes, the sparkplug # 5 was broken, yes, the new one, Replaced it and everything is great again, thanks everyone.