my prosport has been doing somehting similar. when im cruising the boost will slowly creep up to 5 psi and drop back own to 10 inhg vacuum. The needle will just start moving up and down on its own when the cruise is set on level ground. Unfortunately prosport won't replace the sender because i've had the gauge longer than a year.

did they warranty the sender or did you have to buy another one?
good luck getting it solved.

 

the sender was only $20. i haven't put the new one in yet or driven it yet today. i put the sender on a stud that is located just to the passenger side of the master cylinder that had nothing on it. so i slid it onto the stud and just put a nut over it.

 

Thats where i mounted mine. the only problem i have had with the boost gauge is it reads high, at wot it will peg out to 30psi. I Have the cobb ap and it shows my boost hitting its target boost. Does anyone have that problem?