Light string partial malfunction

I bought a no name string of 100 indoor/outdoor lights from Walmart, which is actually 2 joined strings. They worked fine for a couple months but then a section at a time wouldn’t light. I replaced the string at least 3 times & the same thing would happen. Funny thing is that the sections that went out would sometimes work again, without my doing anything. Seems like there is a design problem if the same thing keeps happening with different strings. There is no manufacturer named or ability to contact them so all I can do is keep exchanging them, which is not a viable solution. Any comments or ideas?