1995 Chevy Camaro High Idle RPM

Engine Mechanical problem
1995 Chevy Camaro 6 cyl Automatic 150000 miles

Recently, I started my car on a cold morning and the rpm instantly rose up to 3 (3000).
I let it run for about ten minutes, to let the engine warm up; however the rpm was still at 3.
When I shifted to drive or reverse, the car would jump.
I drove it to my destination and while I was driving, I noticed the rpm was running at a normal of about 2.
However, when I let go of the accelerator, the car would just remain at a constant speed. I even had to brake extra hard, to prevent the car from going forward.
Later in the afternoon, when it was much warmer, the car would still idle at 3000 RPM