Transmission hunting in 4th gear

Started happening right after my buddy changed my plugs and wires, like the very day. He's not really sure. That was like 8 months ago. Eeek, I know, I wait on things; it's gradually starting happening more, but no new symptoms. The shop just told me I need a new transmission, but from what I've read it doesn't always mean that.

2000 year, 3.1L, 160K

Almost sounds like the Torque Converter Clutch is not working properly, but I'll defer to the "gurus" of the forum to help narrow it down for you.