You have one wire in the tamper return for the bell so I take it that either you have wired the tamper returns in series, or only connected one tamper return. You have two negatives in the bell power terminals and the circuit tampers are not wire correctly which would suggests to me that you probably havent wired the bell return in series at all, and probably only have one bell tamper returned. The current being drawn from your system when in alarm is higher than the power that your control panel is able to supply.
Before you go and waste money on buying two new bell boxes.
1. Do you understand what series means?
2. You need another power supply to run everything that you are trying to run off of your panel. At the moment you are trying to tow a caravan with a 50cc motor bike, you may be pulling it along okay but you are putting enormous strain on everything, and eventually something will give.
Your transformer should be around 19.5vAC the 12dc output should be 13.5Dc so I would say the panel power supply is on its way out, or the battery in your multimeter needs changing, assuming its calibrated.