Eh, you're both kind of right really. You touched on it a little bit; you don't need to go WOT in 5th unless you're at a pretty significant speed. From a common interstate speed, WOT is a bad idea in fifth. This is even more relevant to the 16bit ECU as it doesn't have per gear compensation.
And most cars are tuned for 3rd/4th whether it's a dyno tune or road tune. A car tuned for solid XXPSI in 3rd/4th will usually overshoot the boost target a bit in 5th on the stock boost control management.
@OP: Oscillating boost is an indication that you're overshooting your boost target. Talk to the original owner and find out who tuned it, or if it's even tuned at all. Then talk to the tuner. If it's not tuned, then yeah really talk to a tuner, and stop going WOT.
If the cars tuned, it may be because of the colder weather and you may just need to loosen the wastegate arm a little bit. Get more info on the car first though.
Is it just the COBB ots map?
Here's a good rule of thumb. WOT on the interstate? drop it down to 4th at least. Not enough gear in 4th to downshift and go WOT? You're already going too fast!
4th gear tops out north of 100mph... Hell 3rd gear can get you to something like 80 if i'm not mistaken. Plus, EGTs start to be come questionable on most tunes after a hard 4th gear pull anyway.