Auto Air Conditioning & Heater Repair: a/c, nissan altima, ground signal

Question
I have a 1997 Nissan Altima,and the problem is that tha a/c sometimes stop cooling and just blow warm air for a while and then starts cooling again. The mechanic said it coul be the relay. What should I do?

Thank you

Answer
you must have it checked when its not working  a lot of tmes on those cars i see that the clutch gap on the front of the compressor is just too wide to engage  or possibly the ground signal to the relay is intermitently is droping from the cars ecu computor