strange boost on UrS4

[Bob]

A friend of mine is having strange boost problems with his Urs4 (94). He
said on the hiway one day it started to 'buck', then didnt have as much
power as usual. Boost gauge wsant registering much.
Next day all was normal, then eventually went to low boost conditions.
Today he drove it and mentioned it had too much boost, although he liked
the power!  Then eventually went to low or no boost again...

Any suggestions appreciated!
Thank you!
Bob