Hello WRX!
I'm sure you'll receive a few responses from the front desk boys n gals in due course, but before they point the finger I'd suggest that it can often be down to ATC.
There are several factors affecting the flight shortly after departure. Once they've wrestled the beast of the black-top they have to cpmply with minimum climb gradient requirements for noise/terrain clearance whilst also minimising noise produced by the aircraft! Once they get to between 3 and 6000' their levelling off/sporadic climb is down to ATC.
We give climb to an a/c as soon as possible but due to the conflicting traffic this can, unfortunately, be rather eratic step-climbs of 1000' at a time. This happens especially in the evening when the long haul departures come lumbering out of Heathrow and Gatwick. We do what we can to minimise these but sometimes it's inevitable, we'd much rather your departure climbed continuously and got 'out of our hair' as soon as possible.
I'm sure you'll get more responses but there's a starter for 10!