My veritas alarm goes off when unarmed.2004 year?

Hi Just recently my alarm goes off when unset but only in the house.. the tamper light is flashing.. It seems to occur when a high wattage consumer is used. It then goes off every 10 minutes then stops even after consumer has been turned off. I have walk tested all pirs and seem ok and also replaced the battery on the main box. Any help will be gratefull.
Thanks in anticipation