stopping when ahead?

[sweeng8]

I have an annoying urge to stop when Im ahead, especially when its quite a bit. Though obviously this isnt as bad as it sounds, it isnt sensible. Say I have 3 spare hours to play multitable cash games- If im up say 2 or 3 max buy ins after one hour then i feel like i want to stop. Because of this my hourly average is massive, but only because i only play a couple of hours at a time. When i do try to keep playing i never seem to make any more, and sometimes lose some of it. If I could maintain my hourly rate over 4 or 5 hours I would be a happy man, but for some reason I cant. I dont think its a concentration issue, more I dont want to lose what ive earned. This doesnt effect me when i sit down to start, and i feel compelled to keep playing if im behind.

Anyone else have this issue?