I think paddy might be onto something with U3 - it looks burnt in the photo and if it is an optocoupler it could work intermittently.

Other things to try - look at all the solder joints on both sides of the board, if any look loose or crystalline (dull gray instead of shiny) they might be cold solder joints - judicious use of a fine-tipper soldering pencil and some good electronic solder can work wonders for intermittent problems.

If you don't have a soldering pencil and could use one occasionally, you might try one of the butane ones - they are cheap, heat up fast and portable - no wires or batteries. <a href="http://www.radioshack.com/product/index.jsp?productId=2062753:>Radio Shack carries one.

Good luck - things like this can be almost fun trying to track down.