The problem with TSI and Youtube has been getting progressively worse over the last few months and mostly during peak time. They've likely been running out of capacity for a while but apparently just added 30Gbits/sec of capacity to Google/Youtube which was supposed to fix the problem but hasn't yet.
You're likely going to need 10-15mbit/sec to stream 1080p tho without buffering.
there's a dedicated thread for how bad TSI is right now with youtube. it was apparently supposed to be fixed yesterday but the problem still continues. you might want to watch the thread before switching if youtube is important to you.
regardless, I don't think 5mbit is enough for 1080p without buffering, you might want to look at a better package or move to Cable.