VIRT
Verify that a problem exists
Identify the problem
Repair the problem
Test - to ensure the problem is solved
Yes, I know you're looking for specifics to help you identify the problem, but you have given us very little to work with.
Slow speed & intermittent disconnection - most likely result from a congested network, and that can have many causes.
Tell us a little more - describe your network, and we may be able to help you identify the problem.