Doesn't sound that good mate. I have just gone through the HD dilemma myself. Disk utility did not report the fault when I ran a repair permissions sweep.
I was having the exact same probs as you were describing so if you can - I'd backup all your important stuff immediately.
I decided to download
SMARTReporter. It informed me of the fault. Unfortunately it was too late to do anything as I was having the symptoms for about 2 days and the drive was cactus.
Good luck!
aussie_geek
edit - SMARTReporter is installed on all my Mac's. I have configured it to check the drives every 300 mins so I will at least get some warning