It sounds to me as if your tongue is in the wrong position- too far back. Can you make the sound correctly when you try on your own without any pressure?
I mean the pressures of external forces- having an audience, etc. Often people find that when they are in situations where they are less relaxed, such traits get exagerated.
oh yea. when i think about it (pronouncing) is when i mess up most it seems. it's weird, my speech changes depending on who i'm talking to...and also with my energy level (i mess up more when i'm tired)