My overdrive light in my 1999 mercury mountaineer is blinking on and off repeatedly as i drive.?

When i'm at a dead stop, foot on the brake, the light turns green and I step on the gas, it takes a minute to catch. When it does it seems to be jumpy. Also when driving the rpms gage jumps as if I had suddenly pressed the gas. I just bought this ******* truck used less than a week ago and it was fine, now im thinking problems right? But that wouldnt explain everything else would it? somebody help a white boy out would ya!