AC working off and on

[northshore2ss]

Hi,need advice before making another service apointment.Bought car in Jan.this year,in March,during a rare hot spell,I noticed AC not working (blew cold air,then hot) I played with switches,read manual again,still blowing hot air.Brought into dealer to look at at,after waiting 3 hoursInformed," everything fine,blowing ice cold air, we could not find any issues."AC has worked OK most of summer,(ice cold most of the time,but sometimes takes a long time to start blowing cold.)Today ran ice cold in am,now blowing hot air,will not change over to cold after 1 hour of driving.I've read some other threads about line being blocked,but I have puddle of water under car most of time.Just want to have some ideas to talk about with dealership as problem is off and on,and i dont want to waste another am. if it happens to be working when I bring it in.Thanks!