I have a 2006 2.4L P.T. Cruiser (conv) that has 44700 miles. I have a check engine light that points to the problem being the transmission control power unit undervolt.

For 2 months I did not drive my car. I
started the car every week for the
battery charge. 2-3 weeks before I
had finally had the $ to go get it
checked at Chrysler I went  to start
the car and the battery was dead.
After being jumped I saw they the
check light was off. I drive for over 20
miles and nothing. About 3 weeks
later it came back. I disconnected
the ➕terminal for a few hours and
when I reattached it the check light
was again off. The next time it was
abut two weeks and now about 5
days. The transmission runs great,
sounds great and feels great.
Besides spending a boat load of
cash is there any other way to fix this
for good?  Why is it happening and
why does draining the power work?  
Please help. I am planning a long trip
in the spring and I don't want the
specter of a check light that could
happen at any time. Is the module
expensive to replace??