Bicycle Mechanics - Chains

In the last week Precious's chain has come off three time. Twice yesterday. I still have grease all over me...

Quick technical question: is it a fluke of nature or is there something wrong?

Need input:
What kind of transmission system are you running.
How old/worn out is it?

Everything is Shimano 105. The Derailleurs are GS and the shifters are STI. The Crankset is a 52/42/30T. Freewheel is 13-26T. I hope this is what you mean by transmission.

I have had Precious since Oct 2001. My best guess is that we have travelled 1500 glorious miles together.

It happens when going from the 30 to 42 crankset. It seems to jump off track, but not everytime.

Sounds like a limit screw adjustment. On the front de. you have a H and a L number 1 phillips head screw. Place the chain and der. in the outer ring. Turn the L screw in 1/4 turn increments then check to see if the problem sttill exists. If so repeat. If this does not work write back.

But it could be the Front Derailleur is not parallel to the chainrings any more. The arms that move the chain on the FD must not be at an angle to the chainrings. Loosen the clamp and bring them back in line, being careful not to let it slide up or down on the seatpost.

If all is well: Prop up the back of the bike and while turning the pedals by hand, shift the FD and watch what it does.

It probably has nothing to do with the chain UNLESS there is a twisted link. This would be very obvious as you turn the pedals.

It is not the limit screws, if the failure is from small to middle. These stop the chain from falling off the middle and large rings. It may be a non-parellel front mech.
105 is a fully indexed system , it takes one click to make the move. The click may not be trimmed correctly. There is a way to trim the shifting so it places the mech in the correct position (ie further outboard). Refer to your instructions or to the Shimano page ->road cycling->tips-> adjusting the trim

http://www.shimano-europe.com/cycling/

typo: limit screws stop the chain falling off the outside of the large ring and the inside of the small ring.

My apologies for I obviously did not completely comprehend what I read. However given that now I have it would seem that if the rider had a "non paralell front mech" that he would have noticed this 1500 miles ago.
There is a host of reasons why this problem exists. It could be something as small as cable tension. It could also be a case of a few bent teeth on the ring. It could also as previosly stated be a bent chain. It could also be that the inside cage of the front der. is bent in. Or it could also be as previously stated a mis alingned front derailleur.