I have Mazda3 Sport GT 2006 MT, 155 000Km (roughly 100 000 miles)

Bough it brand new in 2005. In May, the check engine light came on with no symptom.

Later I noticed that the cruise control light was resetting itself (turning off) every time I'd turned the car off... keep in mind that I always use the cruise control so I never really tuned the switch to OFF. Then the idle started to become unstable, when the car was cold or warm.

I got it check on a small professional portable device and no codes or problem at all were diagnosed.

Then, during my week vacation, my Mazda3 was in the drive way doing nothing, and the alarm came on, at night of course, very occasionally according to my neighbor. After 2 days, he got my keys, started the car, moved it and parked it and the alarm came on again later. He had to unplugged the battery until I returned.

Obviously it looks like a computer problem, or circuit board. Any clues on how to fix it ? I'm starting to think I will have to replace it, if there anywhere else than Mazda to get it from ? Can I rely on a used one ? Or is it something I shouldn't be messing with without the Mazda people ?

THANX

UPDATE
After plugging the battery back today (June 10th) the radio and the ceiling lights are not working although the fuses are. The doors can be all lock at once using the key in the driver door, but nothing works with the remotes !!!