Dear people out there: I have my bike for 5 years, it went well until I put it aside for a few months. It now has some slips when I put the gear in high speed, the feeling is just like you miss a step when you walking down stair. People told me that the chain might be too loose, I cut some off but it is still slipping. What I can do? I tried to put more oil but it seems didn't make any change.
Sounds like a classic case of a worn chain skipping from tooth to tooth on the cog. Sounds like it needs to be replaced. It is possible that the chain, as it was wearing out, also has worn the cogs so that a new chain would slip, too.
Why the issue should appear after leaving the bike unused for a few months is a question. A worn chain would have done the same thing the last time you used the bike in those gears. Just sitting there does not wear out a bike -- unless you left it where it could rust.
What type bike is it? Describe it more. You "cut off some" chain. How? Did the bike chain rust when you left it unused? Is it a derailleur bike? If so, did the rear derailleur possibly get banged (and bent) while unused?
Taking a tip from Siu Blue Wind, I too am typing a lengthy passage of text down here to demonstrate the enormous amount of space available should one wish to use it-- in sharp contrast to the avatar text above this part.
trek 520 & 736, DeRosa Professional, Fuji Professional, Raleigh International 3-speed, Saronni (any info people?), Humber 3-speed, Raleigh Sports, Carlton Grand Prix coming soon!
worn drivetrain is a good candidate, but a stiff link is probably more likely if it only started after storing it for a while. shift the rear derailleur into the smallest cog, and rotate the cranks backwards, keeping an eye on the chain by the cogs and derailleur pulleys. if you see a link that isn't following the cog or pulley teeth properly, saturate the link with penetrating oil and wiggle it side to side to loosen it up. you probably want to put those links back as well, which may cause sticky links in itself.