What could cause an irregular climbing pattern on takeoff? Yesterday (Dec 21, 2015), I was on board Ryanair flight FR4966 from London Stansted (STN) to Lamezia Terme (SUF). The aircraft was a Boeing 737-800, registration EI-EFW.

On takeoff, the climbing pattern didn’t seem very regular to me. It was clear that the engines were first running at full power, but then decreased their thrust and our altitude seemed not to increase as it would regularly do. I was certain we were about to return to STN due to some kind of malfunctioning. However, we reached our destination flawlessly.

Later, I checked the climbing pattern, and my impression was confirmed. You can see below that the climbing pattern was initially steep, but at an altitude of approximately 5,700&nbsp;feet, the aircraft stopped climbing and proceeded at the same altitude for about 5 minutes.

My question: what caused this steady situation? Did the aircraft undergo a malfunctioning that the cabin crew was able to cope with? Or was it just due to the fact that we were overflying London — a congested airspace — and that was our assigned altitude?
The brief period of leveling off is not unique to the flight on that day. Looking at track logs for previous days, it always levels off at around 7,000 feet for some period.
As Terry commented, this is most likely to deal with the congested airspace. Other routes coming into and out of the London area probably pass through the same space, and keeping flights departing Stansted at a lower altitude prevents them from interfering with other traffic.
Looking at a standard departure chart, there is a warning box:

WARNING
Due to interaction with other routes, do not climb above 5000 until instructed by ATC.

This looks to be the route followed by this flight, at least to the DETLING VOR, so it would be expected that ATC will need to limit the flight's climb until clear of other traffic.