I feel your pain, because I have also suffered with bad instrumentation. They work fine for a while, then start having issues.
About USB scopes------I've also had painful experiences with them. The most common problem is quirky or unstable software that freezes the computer or simply does not respond. Also the analog front ends leave a lot to be desired. If you overload them it may take eons to recover.
Another quirk with USB based scopes is that some computers inject a lot of noise from the USB port.
My recommendation:
If you want to go the USB scope route, stick with the better brands. A lot of people like Pico scopes, for instance.