Plane: Cruise Mode Throttle/Pitch Control Bug

#### Issue details

In CRUISE mode, the throttle and pitch control can get into bad oscillations. In other auto modes like RTL, LOITER and AUTO this doesn't happen. Different throttle values can be used to nudge the cruise airspeed which will sometimes bring it out of the oscillations. The values that oscillate and the values that work are random and vary constantly.

I believe this is a serious bug as the extra throttle pulsing and in particular the constant servo jittering caused by CRUISE mode could easily overheat the servo or cause it to fail leading to an out of control aircraft.

Discussion:
http://discuss.ardupilot.org/t/throttle-and-pitch-auto-control-issues/12081

#### Version
I've encountered this problem with every version from 3.4 (first I tried on Pixhawk) up to 3.71

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[X] Plane
[ ] Rover

#### Airframe type
My Twin Dream

Tridge may have been able to repeat the issue with Cruise mode on his Parrot Disco as well.

#### Hardware type
Pixhawk
NEO-6M GPS + MAG
uBlox CN-06 GPS
Digital airspeed sensor (Also tried analogue airspeed sensor)
Mauch current sensor
Turnigy 50090MG Servos (Also tried HiTec HS-65MG)
EzUHF control

#### Logs

You can see the issue on pitch and throttle here just after 41min, just before 44min and 45min:
https://www.dropbox.com/s/6ictr9d62wh9tvo/10.BIN?dl=0

On this log (10.BIN) RTL is engaged right after using cruise to show that the throttle and pitch work properly in RTL mode.

Other logs:
You can see the issue on pitch and throttle just before 2min, 4min and 9min:
https://www.dropbox.com/s/vtksvwl4jstnvca/11.BIN?dl=0
You can see the issue here from 8min through to 16min:
https://www.dropbox.com/s/l76sh5y2m33st9t/12.BIN?dl=0