Was just looking on another, somewhat less sophisticated, 1800 forum, and there were tons of transmission problem questions. Yet nobody seemed to volunteer the the first thing you should always do with these cars when things don't shift properly. I learned the hard way.

Had the car, think it was a 122, up on jack stands, ready to pull the transmission. Had the drive shaft out and was ready to undo the trnasmission when my mom came out of the house and asked me what the problem was. I told her it was something in the transmission. She suggested that maybe it was just "the shift lever thingy." Mostly to demonstrate to her that she knew nothing about things mechanical, I crawled into the car, pulled the boot and reached down with a big adjustable wrench to undo the shift lever retaining cap. Sure enough, it was way loose. Tightened it up and the problem disappeared.

Seems to me in my 25 odd years of 1800 ownership, this happened to one of them along the way as well. If you have a shifting problem, take my Mom's advice. Before you panic, check the easy thing first.