Definitely sounds like there might be a heat issue.

A useful trick from my 'making the design work on the manufacturing line' days... Pop to maplins, get a can of freeze spray (or B&Q pipe repair freezer) - its a bit pricy (£14 B&Q) but you only use tiny amounts. direct the spray onto hot parts one at a time, small bursts, till you find which component reacts. A temperature probe (Maplins £19.99) is also useful. Obviously adding a fan would help but sometimes replacing a part with a better rated part is a more effective answer, or adding localised heatsinks. Classic examples of poor design are the lack of heatsinks on voltage regulators so they flip in and out of thermal limiting - doesnt stop the circuit working but causes odd side effects as the regulator current limits and the 5v supply rail drops out of spec for a millisecond or so.