Okay so I purchased a 02 blazer 4.3 auto 4x4 with a bad transmission. My buddy had a good used trans out of his blazer that I got cheap, installed it and shifted smooth for a few weeks. Earlier today I was driving to the gas station and on the way back home the Rpms started jumping up and vehicle was revving like it was in neutral barely pulling. Speedo is also jumping around like crazy but it had done that every now and then before for a few seconds and would work like normal. Any ideas what it could be? I have read that a bad speed sensor could cause this but I’m not sure about it? Any input would be appreciated thanks.