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.