52 V70 Auto....A/C problem got me puzzled

[silverfoxcc]

When i start up the A/C gives out a good 8 degree C, blast. Then at variable times, ranging from 15mins to over n hour running, it blows warm air. Then just as suddenly goes back to cold. If i stop when it is blowing arm and wait 10 mins it usually, but not always, reverts back to cold on starting.
Now it cant be a lossof gas ,as thishas been going on for a few weeks, so what can i narrow it down to? component beginning to fail, or something thatjust needs tightening somewhere?