Why I can’t record and watch the program I am recording at the same time

I set a recording for a sports event or a movie then wait about 45 minutes to watch so I can fast forward thru the commercials, this is what I pay for a dvr. When I start watching at some point I’ll hit fast forward and it will take me all the way back to the beginning of the recording. This has been going on for years. I have replaced the boxes with new ones which fixes it for a short period of time and then it starts doing it again. Also using the guide I will start scrolling up down and it will go back to the channel I started on. Now if I pause it to answer the phone or something when I hit rewind it won’t stop rewinding when I hit pause. It will continue for a short period of time then go to live tv so I lose what had been saved. This has been going on for years. I can’t get an answer. When I got the service a long time ago I was told I was at the end of the line so your service technician put a booster on it. I’m done.