Slowly and violently shifts gears and a lot of jolting
 

This is an issue that isn't super common but does happen every few months to me. I have a 98 Blazer and every so often it will have a lot of difficulty switching gears. Once it hits about 20mph, as I continue to press on the accelerator, the RPMs will go up but it will stay at 20mph for a few seconds then suddenly jolt into the next gear. It will also happen at higher gears but most violently at that level.

Not only this but at the same time that this happens it also feels like the car is always trying to accelerate. When I'm just cruising it will be almost as if it's on cruise control, keeping at about the same speed when it should be slowing down faster. It will also jolt after I let off the brake pedal without me touching the accelerator yet, like a horse waiting to get out of the gate.

These issues generally fix themselves after a few days but I'd love to fix whatever is wrong so it doesn't happen anymore. Thanks!