The levels in the 70's are ok, the ones in the 60's are low.

Unlike analogue signals where even a poor signal will show some picture, Digital signals have a cliff where once a level is below what is needed you get no picture at all.

If you are already borderline during the best conditions, it would not take much to change atmospherically or some other external influence to make you lose your channels.

Having a low signal level and/or low signal quality would certainly cause issues similar to what you are seeing.

An amplifier would help boost the level, but would only work if your signal quality is ok (boosting a poor quality signal will not really help though).

Peter