Strange chain/chain ring problem

When riding in a gear where the chain is on the middle chain ring at the front and the 2nd - 3rd smallest cog on the rear cassette it seems as if the chain is clicking or catching on the chain ring. It's a 24 speed bike.
What is particularly strange is this seems to happen intermittently. I can ride for 3 or 4 miles without a problem and then it happens. It can be resolved if I pedal hard with the chain on the largest front chainring at the front and the smallest at the back. However, this again only lasts for a little bit longer.
I've changed the chain which has made a slight difference and the cassette and chain ring were done last year and I haven't done that many miles.
Any ideas?
What is particularly strange is this seems to happen intermittently. I can ride for 3 or 4 miles without a problem and then it happens. It can be resolved if I pedal hard with the chain on the largest front chainring at the front and the smallest at the back. However, this again only lasts for a little bit longer.
I've changed the chain which has made a slight difference and the cassette and chain ring were done last year and I haven't done that many miles.
Any ideas?
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Posts
sounds like poor set up somewhere.
Parktools :?:SheldonBrown
Its possible that its rubbing on the front mech guide, which will be intermittent depending on chain tension.
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