When starting, engine reaches warm-up idle, then stalls.

After the engine is started, the idle goes to normal-then the idle gets thready and goes up and down radically and stalls. The problem is getting worse and more frequent; used to only happen every 10 times starting and stalling was not expected- now every time the car is started the stalling happens.
Once the engine is running for 10 mins or more and the engine is warm the problem ceases and the car runs fine.
Had a mechanic look at it; he said maybe the high idle valve? But then changed his mind, he said he ordered a part (a hose) and replaced it- he didn't charge me and it worked for 1.5 months then back to its old ways again.
thanks.