Gears on my moutain bike keep slipping

[spaced]

for some weird reasoin this morning, the chain keeps jumping on my moutain bike. Im cycling along and it seems to come out of gear then back into gear, then out and back again.

Any ideas why this is happening?

[buildersmate]

The deraillers need adjusting because the cams are not positioning the chain exactly over the gear selected at the stick. Its a screw adjustment.

[whiffey]

As buildersmate says, but you might also check for any slack in the cable and 'pull it through' if necessary.

Also, thoroughly clean your chain and cassette with a decent cleaner such as Muck-Off, and use a good lubricant on it. Many people use ordinary oil, but it attracts grit and can end up creating what is effectively a grinding paste on your chain and cogs. Of preference I use a wax lubricant (the best is White Lightning) on a thoroughly clean drivetrain. It is self-cleaning, in that the wax falls off taking the dirt with it. It keeps the chain remarkably clean. You need to lubricate quite frequently, maybe twice a week in wet weather, but it takes about 5 seconds to apply and is well worth if for a clean well-shifting chain.

[EeezyNoow]

There's several possibilities,worn or twisted chain, wear on rear cassette teeth, bent tooth on sprocket, badly adjusted derailleur, rusted or stiff gear cable, bent derailleur arm. Turn bike upside-down and move pedals slowly backwards by hand. Look for twist in chain. If found straighten with two pairs of pliers or adjustable spanners. Check for worn chain by pulling off front chain wheel (shouldn't come off much). Look for bent or worn teeth on front and back sprockets. Teeth should be nice U-shape with square tops (not sloping or with sharp pointed tops). try changing gear by hand simply by pulling gear cable slightly - you will have to move pedal forward to do this.