I have a Giant Escape, with Shimano Acera gears (3 speed on the front, 7 speeds on the back). When the chain is in the hardest gear on the front cassette, then it is reluctant to shift into the hardest gear on the back cassette, otherwise it works fine.
When shifting from 6th to 7th, it stays in 6th while making a rattling sound for anywhere from 1 second to a minute, before changing to 7th.
I've tried decreasing the gear chain tension until it is basically slack when in 6th, but this doesn't help. I also don't think it is the limit screws (it was working fine before) and it shifts fine when not in the hardest gear on the front.
Also, this started when I had the rear wheel replaced 1 month ago.
This only seems to happen when I'm riding the bike, when it is stationary and I am pedalling by hand, everything works fine, for some reason. Also, I've tried replacing both the inner and outer gear cable.