U2008 regulator triac blows up

[Alana]

I made a regulator box for my AC loads using U2008 regulator kit
.
It works fine when i test it with a light bulb but blows up when i connect my vacuum cleaner. Not as soon as i connect load but after like 5-10mins of working at low power setting.
At first i used BT139 triac then i replaced it with beefiest triac i could find BTA41. Both blow up with the same faliture - shorted gate to A1.
Recently i added a snubber circuit - no luck, another triac blown. Triac is mounted on a radiator in ventilated part of the case.

In this configuration it worked fine with smaller vacuum cleaner and circular saw. Motor power 1000W and 700W. Big vacuum cleaner is 1400W and this should be well within even BT139 load current capabilities, but it blows.
I suspect there may be something with voltage spikes that kill my triac but I have no means of safely scope it at the moment.
Any advice?