2005 PT Cruiser Touring Manual Transmission

I brought 2005 PT Cruiser Touring manual
transmission with 108,000 miles two weeks ago
from Dealership and it ran like new for the first
week. Now engine is randomly stalling while
driving anywhere between 10 to 30 minutes
driving. It happened on the highway at 60 mph and
on the street when slowing down to stop at red
lights. Sometimes it starts back up immediately
other times after a few minutes. I took to pep boys
they said they couldn't recreate  problem & soon as
I got in car to drive it stalled in parking lot. They
kept it overnight and told me computer needs to be
flashed by chrysler dealer.  Took to dealer they
charged me $125 for diagnostics and told me they
couldn't recreate the problem when testing driving
but I needed new timing belt, water pump etc. for a
total of $2200 I told them the car runs like new,
engine purrs and the only problem was random
stalling.  They insisted it was timing belt so of
course I declined the work and left.  The only other
thing I can think of is that I had a aftermarket radio
installed.  Could radio cause engine to stall or does
computer need to be reset like pep boys
suggested? Or what else could be the problem?