2001 Chevy Blazer 3rd and 4th gear

I drive a 2001 Chevrolet blazer automatic, rear wheel drive with 113k miles on it. I was driving normally one day and all of a sudden the transmission would slip into neutral due to the safety feature at about 3rd and 4th gear. I got it checked and was said the 3rd and 4th gears are gone, I buy a transmission with 54k miles on it and swapped it out with the old one, it drove well for the first 100 miles. The problem with the first transmission came back and I can't figure out why it would be doing this. Coincidence that the new transmission is shot too? Yet it was working fine for 100 miles or so after being swapped? The engine tempurature gauge reads at about 140 degrees instead of about 210. The gauge went down to about 140 when the original transmission went out. Also after I drove it back home I could smell a sort of electrical burning. Engine oil was just changed and so was the transmission fluid prior to putting the new transmission on. I can't figure out what it is that's wrong.