Is My Mechanic Treating Me Fairly - Is This His Problem or Mine?

I have a lovely 1990 Chrysler New Yorker in very good condition with
80,000 original miles. In April of this year it had approx. 65,000
miles and I had to have the automatic transmission completely rebuilt
for about $2600.(canadian), but the work was done in California. The
warranty was 6 months or 12,000 miles and had just expired. This week
I noticed that on rare instances (once or twice in 300 miles) the
automatic shift into low gear didn't seem quite as smooth so I decided
to have it checked. I took the car to a Chrysler dealership, explained
my concern and requested that they check things. After two hours of
labour they advised that the mechanic could find nothing wrong and had
"reset the codes".
I picked the vehicle up and drove it less than one mile when the
transmission started lurching very badly (enough to throw us forward
against the seat belts). It did this approx. 5 times in 100'. I drove
one more block to the house with the car placed manually in low gear
and then called for a tow truck to take the car back to the
dealership. I am now being told their transmission specialist (not the
mechanic that did their original check and "code reset") had rechecked
things and decided that I now need a new transmission.
I am having a hard time with this explanation. The transmission
had been working okay until I took it to the dealership (with the
exception noted in paragraph one above). They work on it for two
hours, give it back to me and now it is totally undriveable after
driving it for less than one mile. I can't help but feel that their
mechanics "tinkering" might be the problem. Is that possible? I would
like to hear some opinions and advice on this problem, particularly
how to deal with the dealership.