I was driving down the highway and I heard a "braaaaaaa" noise coming from my truck ( 1999 Chevy Silverado Z71) it sounded from the engine and I was on cruise control going 65mph all of a sudden my rpm meter shot up from 2000rpm to 4000rpm for about 6 seconds and I start slowly slowing down and my engine light started blinking then I tapped the brake and it went back to 2000 rpm then I turned back home and nothing happened going back any ideas? It has 218,580 miles
It sounds to me from your description, the transmission is going out of your truck. Either it lost line pressure or the clutches just weren't holding. The constant pressure of running the truck via cruise control overwhelmed it or something, who knows. This will more than likely happen again and when it does, you probably won't be as lucky to get it home. The likely scenario is, when you released the engine from cruise control, this gave the transmission a chance to reset and "get a grip" on the bands or clutches, whatever was slipping at the time. Once re-engaged, the transmission was able to start functioning normally again.
You could possibly check the transmission fluid to see if it is low, but that's a long shot. Low tranny fluid could have allowed the pressures to drop and for it to start slipping. Remember to check the fluid when the truck is completely warmed up. You're not only looking for level, but to see if the fluid is burnt. More than likely it is burnt at this point, but that's a guess on my part. If it is burnt, you could possibly get away with a fluid change, but this would be a short lived solution. If you've never changed the tranny fluid before, this can become problematic for a high mileage transmission. It's a long story, but trust me here, you'd not really be doing your transmission any favors (counter-intuitive, I know).
Your truck comes equipped with the 4L60e transmission, which is an anemic choice for a full sized pickup, especially if you want to do any towing with it. Why GM used this transmission in a truck application is beyond me. If you have 218k miles on your truck and you've not had issues with it before, I'd consider yourself lucky. Plan to get a new transmission or get yours rebuilt in the near future. I'd suspect it doesn't have long to live.