There are several things that could be causing your problem as follows:
1. Auxiliary power failing. Loose fuse or poor connection at the panel down powering the vipers will cause the circuit to go open giving the impression that they had activated. This is why its important to know If the pir were to activate along with the vipers when the system is fully set as this would indicate that it is a power problem.
2. The cables to the vipers may have a fault on them, the fact that you have nt had a tamper fault would suggest that the 24hr loop is okay, if you swap the pair both ends and you start to get tamper faults then you know its the cable. Although it could also be an intermittent short between the tamper and alarm circuit. So if the false alarms continue it still maybe a short between alarm and tamper, To prove this I would remove the wires from the tamper circuit and fit a link, if the fa's stop its a short problem.
3. The panel itself. It could be just the zone or it could be dropping power
4. The vipers. As there is two of them it is very unlikely, but not impossible.
My money is on 1 or 2 the detectors seem to work as they should when tested and the panel operates correctly, but an intermittant fault can be difficult to find especially without a multimeter.