power relay on scholtes hob cuts out?

intermittently, touching power on sensor either gives no response or it turns on but as soon as you start a ring it cuts out. Have had engineer out 3 times - replaces relay & it works for a while then same fault. So a few times I have opened it up and opened and inspected the relay, done nothing, screwed it all together and - hey presto, healing touch, it works again for days weeks or months. It also cuts out with mild physical trauma, but wife finds she can sometimes fix it by hitting it!! But gets frustrated if this doesn't work & she (we) can't cook!
Model is THN5600 - expensive so would like to fix rather than discard or waste money on useless engineers. Can anyone Help? Charlie B