It does sound as though there could be an issue with loose connections in the sensor, wiring, junction boxes, etc.
Is the sensor wired and definitely on its zone, and not looped in series with window contacts or shock sensors? Contacts can go open circuit if the magnet is mounted too far away or has gone weak or the reed switch is faulty, and there's vibration.
I would try covering the sensor with something to shield it so that it doesn't pick up body heat (e.g., a piece of card taped on). Then try tapping the case of the sensor to see if it triggers. The tamper switch on the PCB may not be held tightly closed with the cover in place. I had an issue like this with my system and tapping the PIR triggered an alarm. It turned out that the little piece of rubber on the front part of the sensor didn't push tightly down on the microswitch with the front replaced.