Picture dropouts on HD cable channels is a very commonly reported problem. For the vast majority of people it happens very uncommonly. For a few people it happens commonly but is still tolerable. When it happens every 4 seconds, you have (a) a very uncommon problem that is (b) clearly intolerable.
The first step in solving the problem is to do what you have already done - get the cable company's tech to take a look. Not infrequently the problem is due to a poor quality cable signal - and presumably the Rogers tech checked that issue for you, and eliminated it as a cause. Assuming your Rogers tech did his job thoroughly and properly, then you will have to take his word for it that your relatively older model cable box is at fault. If you do go ahead and buy a new one and you still have picture problems, you can at least throw the problem back at Rogers to fix until you are satisfied.
If you are going to be subscribing to Rogers for at least the next two years, it makes more sense to buy an HDPVR rather than rent one. Despite the problems you are having with yours at the moment, these boxes are usually fairly reliable over the long term.
