I am no serger expert by any stretch but it sounds like the tension might be off on one or more of your threads?? Or a bit of thread/fuzz/lint somewhere in the thread path?
When I bought my used serger I took a class as I'd never sewn with one previously. Best piece of advice in that class was to have a notebook by your side every time you use your serger and note what your tension settings were before AND after each project. Along with type of fabrics used and the project itself. Always write down your starting point and always return your machine to those settings upon completion of your project.
Other than that I have no other suggestions but I'm sure someone else will be along soon with better suggestions.
Good luck.