Vacuum sounds right, as for boost...dunno.....is there a gain setting on your profec? If so, try messing with that. I forget which way it goes but if it is too far one way it will spike and then drop, too far the other way and it will surge and vary.
How high are you talking about when you say top of the gear? I have heard that the stock wastegate actuator can cause the boost to drop up high. To combat this shim the wastegate with one or two washers. This will help hold the wastegate open for the last few hundred RPMs.
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