Comcast HD freezes and Pixelates on Premiere

I have Comcast. Although I am in Delaware, I am in the Philadelphia market. My Tivo HD with two single stream cards works great, but the premiere with a multi stream does not. I have replaced premieres as well as cable cards. My pictures freeze for maybe 20 to 30 seconds, works for 2-5 seconds and then freezes again on HD channels and pixelates. They fixed the problem about 6 months ago and now it is not working again. They gave me a DVR for 12 months at no charge, but that has only a 250 gig hard drive. They put an amp in line and I am getting 90-95% signal strength on the premiere. The HD unit has a built in amp and gets 95-100% signal strength. Any suggestions?