Q: Intermittent performance issues

My car jumps for a few seconds when I'm cruising. It doesn't typically happen at lower speeds that I can recall. I notice it more when I'm driving on highway. It lasts for a few seconds (maybe 1 - 5 seconds) and then goes away for awhile, then comes back, goes away, etc. It feels kind of like letting off the clutch a little too fast. I'm not on the clutch at all when it happens.

Although, if I turn my car off, even for just a short while, it may not do it through the whole trip until the next time I turn it on. It's as though each time I turn my car on, it decides whether or not it will do this "jumping" during this trip. It happens maybe a third of the time. But when it does, it doesn't go away until the next trip (turned off and then on again).

What's going on? Is it serious? About how much will it cost to fix? How long is the procedure?
My car has a manual transmission.