Oncidium incurvum help needed!

[Helen]

Hi everyone.... the last few days my oncidium incurvum leaves are starting to yellow and get soft and drop off.. I lost approx 5 of them and a few more are starting to yellow. My watering has been irratic due to hot then cool weather here in southern ontario....any suggestions?
Am I over or underwatering? I really don't want to lose this one...I'm having so many problems with almost every orchid I own that I'm getting discouraged....at least my new brassia is blooming, so that gives me the 'oompf' to keep at this....!!