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Rev limiter will kick in, however, you should not go into limp mode unless you drop a gear or do something that pushes the revs way too high too quickly.
The stutter you're getting at ~5000rpm sounds like detonation (knock), which is a bad situation. This usually results from a bad air-fuel mixture and the engine management should retard timing to try and prevent it. If the knock sensor detects detonation, your engine may go into limp mode.
The most common cause for this on a turbocharged car is either a boost leak (hence replacing the hoses) or a lousy 'chip' (after-market tuning). If you take the car in to a stealer, they can read the fault codes. You'll probably see stuff relating to the VANOS system.
Simply replacing hoses is not the way to deal with a boost leak. Trust me - I've owned an 11-year-old Audi RS2 where the plastic hoses were tired through years of hot SA summers taking their toll on plastic and 1.4 bar of boost. (The 135i, by comparison, has less than half of that.) There are usually many hoses and many places that pressure can leak. Air can also be sucked in on the cold (intake) side which is even more difficult to detect as the car will still boost strong, just with a bit more lag.
It is also possible that you're experiencing the first signs of HPFP failure. Sadly, this is commonplace on the N54 engine (135i/335i). Look out for the other tell-tale sign, like long cranking (taking more than a couple of seconds for the engine to start from cold).
Also, check the obvious: where are you filling up with fuel? Try use the same petrol station and make sure it is reliable. You'd be surprised how much bad fuel is being sold ( under all brands) so it's more important to stick to a trusted filling station.
The problem here is that you don't have a whole lot to work with, so, if you can replicate the problem, replicate it and drive straight to a dealer and make them pull the fault code history for you and print it out. At least then you know where to start.
If there is anything relating to the high pressure fuel system, get the HPFP replaced. If you're getting boost pressure or air-fuel mixture fault codes, then you need to look for boost pressure leaks.
For boost, drive at about 3/4 throttle with the windows down and listen for whistling. Note, a certain amount of air has to be let out the wastegate, but this shouldn't produce a high-pitched whistle like a leak. Also, check if you're experiencing bad turbo lag. Go into 4th gear at relatively low speed and then floor the throttle. Does it feel asthmatic or does is pick up revs like it should?
What is your average fuel consumption like?
If there is a boost issue (which you need to diagnose first scientifically), don't even bother with the dealer. With my RS2 I tried four different Audi dealers and spent thousands of rands for them to talk bull before a friend located it for me using the simple trick of holding a cigarette above the various hoses and watching which way the smoke blew...
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