We need more information specially about what transmission system you are using - eg 3x7/8/9 deraillier or Hub gears or even a mixture. I have heard of bikes where the transmission has been 3x14 using a rohloff hub and MTB front triple crank... but that to my knowledge has never been made as a standard offering.
You might try checking that the chain is correctly tensioned and also clean and lubricated (crap in the chain will cause it to jump)
Also have the bike on a stand or upside down and pedal in the higher gears using your hands - that will give you an idea of what is happening when the problem occurs which will also give you a clue to why.
If its a deraillier system the control cables might need adjustment because they stretch over time a little and with the 7-8-9 spd cassettes a little stretch can cause all sorts of problems because the amount of shifter movement between ratios is so minimal.
If you are using a hub system (at least a shimano, like I have) you check cable alignment by putting it into the middle ratio on the shifter (in my case '4') and then looking at the selector on the hub itself you are looking for two red dots to be in line with each other... if they are not you need to make the relevant adjustment to ensure that is the case and no more slop.
In the case of the hub system the feeling resulting from misalignment is similar to the effect you get when a manual clutch is slipping in a car.. it seems to jump and judder under load.