Which bomber are you having problems with Reynolds?
Most heavy buffs have a tendency to consistenty climb, even when if supposedly level flight, when at maximum power.
I personally tend to reduce the RPM ("-" key on my number keypad) then slightly reduce the Manifold pressure (MAN.... aka your game/joystick throttle, or the "+/=" and "-/_" keys on the keyboard if controls are still default.) to settings that will keep me at a steady altitude when the speed has steadied up.
Also to help your altitude steady up, level your plane, and make final major course corrections, one sector out/ 25 miles. More with faster buffs liek the Arado and Ki-67's.
If you are at a level altitude and constant speed a sector from target. Take a long calibration (I'll calibrate from 15 to 30 seconds when I can). Check the E6B numbers against the numbers in the bombsight (if they are off more than 1 foot, you are not quiet stable, and should recalibrate.... don't adjust throttle/rpm/Man at this point, you are close enough, and already in the bombsight for the run at this time).
Using this technique, I usally can hit a strat target with a single salvo of 250lb bombs from 20,000 feet 7 out of 8 times.
IJN will have D5N2's, Ki-67's, and Arados..... these don't have as much altitude creep as the heavy bombers do, and usually attack from lower altitudes. I usually attack with these planes as full throttle, unlike the heavy buffs. Still need to be level and at full speed well in advance for accurate calibration.
