well i got an mcx 2 off of someone and it worked perfect for like 10 flights.. then i noticed it was going good.. i had to kill the motor to avoid hitting a child and when i reset it up it was significatly under powered... with my 120 MaH lipo. I figured i just crossed the "threshhold" and it was just dying.... so i put in my fully charged 150 MaH lipo and it performed just as the 120MaH did when "dying".... i cycled through both batterys once more and the heli seems under powerd.. like it can't even stay hovering.. i examined under the canapy and nothing seems out of place or anything! i need help please.. i love this thing
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Does it start to "die" right away... or does it not take off at all...
If something was slipping on the shaft... it would seem under powered.... maybe.
blades not getting the RPM needed.

It's likely your li-po battery (both the 120 mAh and 150 mAh - if neither holds a charge anymore) - both have either gone bad (self-discharged too long or overheated). I am learning the hard way that the Achilles heal of these copters are those damn ultra-sensitive lipos. "Always wait 15 minutes after flight to recharge" "Do not store fully charged" "If stored too long they will self-discharge and die beyond repair" "Make sure they are never stored in a hot location" "Never run your lipo battery down - stop flying after 5 minutes" "Do not under any circumstance stare at a lipo for more than 3 seconds or it will implode", "Blah, blah, blah" (it's ridiculous really). I wish I would have known this before I got into the hobby - and am surprised how few in these forums do too. Please all - correct me if I am wrong. At least the good news is, the MCX2 is likely fine (very well-built and durable product). You will just have to invest in lipo batteries (@$5 each) more often, and try and follow the unwritten rules I have shared. Hope this helps.