i have a 10.8kW shower with a cheapo Wilkinsons isolator pull cord. My shower is used by me on average for about 20 minutes, followed by my wife who uses it for another 20 minutes every day. This isolator switch has been fitted for 5 years and gets turned off after every session of shower usage. My son has a shower in the evenings, so i reckon that in 1 day the pull cord is operated 4 times a day. 2 on's and 2 off's. Ive never had any issues with the pull cord isolator and its checked every 6 months for cable security, etc. I would say something is not right with part of your setup. Whether that be loose connections, turning on and off under load or too small cable i couldnt say without seeing it.