Hi guys,
I'm digging up an old thread for a very good reason.
I've had this very issue with the R34 boost gauge dropping out for nearly 3 years, i finally decided i would muck with the boost sender unit and if i broke it i'd need to buy a new one anyway!
So basically as Jamie has stated above, i am very pleased to confirm that if you open up the boost sender unit and simply re-heat the 3 large solder joints shown in the picture in the first post, heat them so they become soft and liquid-ish and then allow them to re-set again (takes 2 seconds), do this to all 3 of the large solder joints and the simply re-assemble the boost sender unit, and go for a drive, your gauge will infact work (read: 'should')
After going out for a 10 min drive, i am super pleased to say that the gauge has not missed a beat, i'll be monitoring it very closely over the next week, but i am highly confident that this has solved the issue.
So if you have this issue, grab yourself a $15 soldering iron fron Bunnings and simply follow the above steps and enjoy the result that is a working boost gauge in your R34!