1995 Cadillac Sedan Deville 4.9

my blower motor when out I bought a new one blew that one out and now I have another one I plug it in it goes to low to hi back to low and off but why it’s on it pulsates with power up really fast down every few seconds and then when its been running for a little while it goes really high when I turn off the motor it stays on and stops pulsating with power I have to pull the fuse to get it to stop this is driving me crazy please help