Greetings,
What are you using, Legacy, Pegasus, or thales New FM?
IDLE and PERF Factors are data enabling the customization of your aircraft, in order to fine tune the predictions of the FMS.
Because the FMS Perf database is a generic one, and engine degradation takes place.
IDLE factor is provided by Flight ops engineering, and is different from one aircraft to another, it has to be entered during DONE, and PREFLIGHT flght phases, inhibited elsewise.
Now vertical profile of A320 and 340 families, is based on optimum i.e., most economical profile, given CI, WIND, TEMPERATURE, TROPOPAUSE height, and aircraft Weight.
The FMS will always try to keep the aircraft on its Optimum profile, using the Optimum Speed.
So if all the data have been entered and are correct (Rubish IN Rubish OUT principal)then the FMS SHOULD NOT behave the way you describe it, unless it is a old Legacy FMS.
TOO STEEP PATH is the result of constraining waypoints altitude wise followed by a speed reduction/restriction, and the FMS has computed that is cannot dissipate the Energy to loose Speed and Altitude (unless you help him with a liitle bit of speed brakes)
IDLE and PERF factors can be modified by crew, the only problem is that the data you are going to use have to be provided by Egineering, because they impact the Economics of your flight i.e. -XX% will delay your descent +XX% will make yo descend early (on IDLE segments) therefore your fuel consumption will increase
The other thing influencing your descent is the CABIN RATE of DESCENT setting
