Everyone has had a go and noone is answering the question, so I will add my bit.
What your seeing is the unit is being operated below it's minimum part load point for a given condenser temperature, typically the impellors are sized for the design condenser water temperature, at ARI conditions this is 85 F. ent. and 95 F. lvg. the minimum part load for this unit at these conditions could be as high as 20 - 30% depending on the condenser selection.
If condenser relief is available i.e. for every 10% off loading the condenser water drops 1 deg. the unit will operate down to a lower level.
The cause of this surge is due to the vanes being closed off and there is insufficient gass flow to overcome the condenser pressure which tries to go back thru the impellor into the evaporator which is at a lower pressure.
I f you call your local Trane office and give them the unit model number they can give you the minimum part load at design conditions and also the igpl values which is for every 10% of off loading the condenser water drops 1 deg.
IGPL is (Intigrated Part Load Value)
If you cannot obtain condenser relief you need to put more load on the machine, which is not always possible in which case you should suggest a small chiller be installed for when the sysytem is below the minimum load.
Hope this answers your question?