Tachometer fluctuates

When I am driving down the road and I shift into third and sometimes fourth (but not as often) the tachometer will fluctuate in half second intervals from a low almost idle speed to a almost dangerously high speed (3500 to 4000 at least) and the entire car will jerk, or buck, in sync with the gauge. It will buck about three to five good times before finally shifting into gear. When in gear it will run fine. Also if I up-shift before the rpm's go above 2,000 it will shift normally, if there 's not a little delay firs t(I have the option to switch to manual). It does this maybe 90% of the time if I don't up-shift beforehand. My best guess points to busted shift solenoid but I thought I'd ask here first as I'm having trouble finding anyone to open the transmission without completely rebuilding it. Also, I checked the code and on every device (even professional code readers) it gives back an error.