1998 Gmc Sonoma acting up!

Hello, i have a Gmc Sonoma 1998 4.3 liter. It started shifting hard and there was some fluid leaking from the transmission vent. He fixed the leak and serviced it, he said all looked perfect with it. I drove it around and after about 20 min it started shifting hard again, the check engine light came on and the code was P1870 for the torque converter.I turned it off and took it back in, the check engine light was off and shifted and ran fine.. He told me to just drive it till the light comes back on.
Does anyone know what could cause this? also when my truck down shifts at like 1000 rpms and i accelerate the RPMs drop fast then go up to where there suppose to be. He ran checks and tested it but nothing...Email me if you have answers robertnathan09@yahoo.com thanks! P.S. does it sound like the torque converter?

the hard shift is due the valve body most likely. my 99 sonoma does the same thing and i checked alldata for tsbs and one was for hard shifts and the problem was from the valve body. they go out around 20k miles

i have a 1998 gmc sonoma. 4.3. i shut it off and about 30 minutes later went back to start it again and it would just turn over. it wouldnt start. the fuel pump wasnt turnin on. i went and bought a new pump and put it in and its still doing the same thing. the pump isnt turning on and the truck still turns over just wont start.