Another possibility is a poor connection at any of the multi-pin cable/board connectors which you are likely to find inside. Try gently waggling them and make sure they are fully engaged.
As for the solder joints, you would need to examine them under good light with a strong magnifier (at least 3X magnification), looking for hair-line cracks (often in a ring shape around pins of heavier or thermally stressed components such as transformers, large electrolytic caps and power semiconductors).