Recently, my road bike's chain got cut, so i took it for replacement. A new chain was inserted and the shifting had a good transition. However, when i shift on the harder gears, i hear "crack crack noises" as if the chain is not well fit. I returned it to the repairman who still said he now fixed the problem but i still have the same issues. It seems he only tested the bike while suspended but not on actual ground. Is this because the new chain was too long ?
You probably need a new cassette/freewheel. A new chain on a worn cassette will jump (especially on the harder gears).
The harder gears have two things going against them. Less points of contact to share the load. Less points of contact so they tend to be worn the most.
If you have front derailleur then using the bigger ring there will let you use bigger cogs in the rear. If I can be in 3 or higher in the rear on the big ring up front then I use the big ring up front to keep miles off the smaller cogs in back.