First thing I would do is check your basement, if you have one. In almost all cases, at least here in the western U.S., the causes of wall cracks can be detected by inspecting the basement walls. Often, the walls (especially if they are the bare concrete rather than finished) will show where stress is being applied. In your case I wouldn't be surprised to hear that the house is built on a concrete slab, in which case the footings of such a slab would be suspect.
The numbers of cracks that you're describing would be unusual in a house of that age here. Although we experience fairly severe winters with 20 degrees below zero (F) temperatures which causes the house to contract (and later expand) we have few cracks. Ninety degree cracks you describe, are usually a sign of framing movement within the house... even hairline cracks.
I do have to say, that I'm assuming the standard skim coated dry wall finish and not stucco or something similar. The harder the coating the more susceptible to cracking is usually the rule.
Best of luck!