MTV 8000D Tuner Problem?

If watching channel 83 and start recording channel 81 from the preset recording, it will turn to black screen. Then no more channel 82, 83, 84 and 85 can view unless "hard restart".  Fixed around three months ago at Science Park and it occurs again last week.  Any clue for me?

I think it's the tuner problem but the technician said that last time it's related to the quality of the antenna and the quality of the digital TV signal. But he confirmed that some electronic parts were replaced.

I've bought a fan to reduce the temperature of the box and also replaced the antenna cable with a good quality one but it finally occurs again.  Please help.