Tempuature gage goes up and down..also fan is not running properly?

when i drive the temp is good..when i stop or idle the temp goes up but does not overheat..still when it is idle i can rev it a bit and the temp goes back down...i have notice my fan is not working like it should be.. also i believe i may need to flush the rad because there is sludge in the rad and the coolant resivare..could this sludge be the reason fo this problem?......Please any advise will help