Front derailleur slipping, please help

[nappadang]

Just over the last two or three days I've started to experience some pretty bad gear slippage. This tends to occur when I'm a out of the saddle and putting pressure on the pedals. Most of the time this problem causes a jolt and takes about half a pedal revolution before I can pedal normally again, though the chain has come off once. Obviously, this is quite alarming when it happens and I generally lose momentum as a result.
At first I assumed it was the rear derailleur causing the bother but I happened to be looking down as it happened. It happened quite quickly but it appeared as if the chain half slipped from the outer chain ring to the middle (causing the jolt) then back to the outer.
I'd really appreciate any suggestions as to the cause of this. Even better would be some potential solutions.

Thanks in advance,

Mick

 

[Brandane]

So ..... is it a triple chainset then, and the chain is slipping from the largest (outside) ring, towards the middle one?
Sounds like either the cable tension from the gear shifter to the derailleur is slightly slack (so the chain gets onto the large ring but then the derailleur moves back a bit and takes the chain with it. Is there an adjuster barrel somewhere on the cable, either at the shifter or on the downtube? If so, tighten it slightly) OR the limit screw for the outward movement of the derailleur isn't adjusted properly and is not letting it go far enough for the chain to stay on the big ring (adjuster screw needs slackening to allow the derailleur to move slightly further outwards).

 

[nappadang]

So ..... is it a triple chainset then, and the chain is slipping from the largest (outside) ring, towards the middle one?
Sounds like either the cable tension from the gear shifter to the derailleur is slightly slack (so the chain gets onto the large ring but then the derailleur moves back a bit and takes the chain with it. Is there an adjuster barrel somewhere on the cable, either at the shifter or on the downtube? If so, tighten it slightly) OR the limit screw for the outward movement of the derailleur isn't adjusted properly and is not letting it go far enough for the chain to stay on the big ring (adjuster screw needs slackening to allow the derailleur to move slightly further outwards).

thanks for that very detailed reply, I'll give it a go.