The cartridge collar of our Moen Monticello faucet keeps coming loose and when it is loose, the handle does not control the water stream. Positive pressure on the collar lets you screw the collar tight - but it quickly comse loose again. The problem repeats itself every couple of days.

SOURCE: Repair cartridge in Moen Monticello single handle lavatory faucet

After some additional experimenting, contrary to information I was previously able to gather, I discovered that the white plastic retainer doesn't (in this type of Moen faucet) actually screw off but rather is held in place by a spring clip inserted from the rear of the retainer (toward the wall side of the basing). Simply sticking a small screwdriver in the opening (approx. 45 degrees downward) and tipping the screwdriver toward the front of the faucet, snaps the spring clip out through the opening.
The plastic retainer than lifts off. Note: the flat sides on the retainer suggest that a wrench will be needed but such is not the case.

SOURCE: Bathroom Faucet Handle Fell Off

There is a small allen screw in the base of the handle- the part the stem screws into to turn the water to the on position. If you have lost the one for your handle call moen at 1-800-289-6636 in the US. Their hours are Mon-Fri 8-8 EST and Sat 8-18:30 or follow this link MOEN. In Canada 1-800-465-6130 and hours are Mon-Fri 7:30-17:00 EST.

SOURCE: how to remove the handle of a monticello 4551 moen

grab bottom of handle,(the fat part of handle closes to faucet body) and grip hard and turn counter clockwise to get to cartridge. dont use channel locks pliers or you risk damage to finish. can be done by hand and very tight grip

SOURCE: I have a Moen Monticello bathroom faucet. I am

Moving left to right: I am not an expert, but I have installed a fair number of faucets. Seems like a basic issue of just getting under the sink and tightening up. Make sure someone is topside to assure the faucet stays straight and does not move as you tighten up below. It is never easy to get a wrench and your hands in the proper position to tighten up the nut, but don't give up try a smaller Channel lock wrench if necessary.

Sink Stopper: http://www.lowes.com/lowes/lkn?action=howTo&p=Improve/PopUpStop.html

SOURCE: Loose handle on my Moen single handle kitchen faucet

I had a similar problem. After a few weeks the lever began to "tighten up". It began first with the hot water at full flow and progressed over a couple of weeks to full control.