Q: Late 2009 iMac is failing. No obvious reason - help!

My late 2009 iMac has been getting slower and slower in last few months and is now practically dead. I've checked everything (Ram, CPU, rogue processes) and can see nothing obvious. I have 1.8GB free RAM, CPU never looks busy in Activity monitor. I upgraded to Mavericks on hope it would help but nadda.

I keep getting the waiting ball appear when I try to do things. Sometimes I get an hour of undisturbed bliss, then relentless waiting with every click. When I restart the machine it is always at its worse, till about 2-3 hours later when the iMac seems to have settled and I can actually do a few things without prolonged waiting.

I tried to reinstall Mavericks to see if that worked but the reinstall failed at the last minute, leaving me back where I started.

Full system details of my machine are:

Hardware Information: ?

    iMac (27-inch, Late 2009) (Verified)

    iMac - model: iMac10,1

    1 3.06 GHz Intel Core 2 Duo CPU: 2 cores

    4 GB RAM

Video Information: ?

    ATI Radeon HD 4670 - VRAM: 256 MB

        iMac 2560 x 1440

System Software: ?

    OS X 10.9.4 (13E28) - Uptime: 0 days 3:1:52

Disk Information: ?

    ST31000528ASQ disk0 : (1 TB)

    S.M.A.R.T. Status: Verified

        EFI (disk0s1) <not mounted>: 209.7 MB

        Macintosh HD (disk0s2) / [Startup]: 999.35 GB (597.53 GB free)

        Recovery HD (disk0s3) <not mounted>: 650 MB

    HL-DT-ST DVDRW  GA11N

USB Information: ?

    Apple Inc. Built-in iSight

    Apple Internal Memory Card Reader

    Apple Computer, Inc. IR Receiver

    Apple Inc. BRCM2046 Hub

        Apple Inc. Bluetooth USB Host Controller

Gatekeeper: ?

    Mac App Store and identified developers

Kernel Extensions: ?

    [not loaded]    com.Huawei.driver.HuaweiDataCardDriver (2.9.2) Support

    [not loaded]    com.novatelwireless.driver.3G (2.1) Support

    [not loaded]    com.novatelwireless.driver.3GData (2.1) Support

    [not loaded]    com.novatelwireless.driver.DisableAutoInstall (1.0.1) Support

    [not loaded]    com.option.driver.Option72 (2.15.0) Support

    [not loaded]    com.option.driver.OptionHS (3.26.0) Support

    [not loaded]    com.option.driver.OptionMSD (1.21.0) Support

Startup Items: ?

    GT Network Tuning: Path: /System/Library/StartupItems/GT Network Tuning

    NMPCCardDaemonVMC: Path: /System/Library/StartupItems/NMPCCardDaemonVMC

    NMPPPMonitor: Path: /System/Library/StartupItems/NMPPPMonitor

Launch Daemons: ?

    [loaded]    com.adobe.fpsaud.plist Support

    [loaded]    com.sonos.smbbump.plist Support

    [running]    com.trusteer.rooks.rooksd.plist Support

Launch Agents: ?

    [loaded]    com.hp.help.tocgenerator.plist Support

User Launch Agents: ?

    [loaded]    com.adobe.ARM.[...].plist Support

    [failed]    com.apple.CSConfigDotMacCert-[...]@me.com-SharedServices.Agent.plist

    [not loaded]    Internet Everywhere

User Login Items: ?

    iTunesHelper

    Dropbox

    SPanel

    HPEventHandler

Internet Plug-ins: ?

    Flip4Mac WMV Plugin: Version: 2.3.2.6 Support

    FlashPlayer-10.6: Version: 14.0.0.145 - SDK 10.6 Support

    Default Browser: Version: 537 - SDK 10.9

    Flash Player: Version: 14.0.0.145 - SDK 10.6 Outdated! Update

    AdobePDFViewer: Version: 9.5.5 Support

    QuickTime Plugin: Version: 7.7.3

    OfficeLiveBrowserPlugin: Version: 12.3.6 Support

    Silverlight: Version: 3.0.50106.0 Support

    iPhotoPhotocast: Version: 7.0

Audio Plug-ins: ?

    BluetoothAudioPlugIn: Version: 1.0 - SDK 10.9

    AirPlay: Version: 2.0 - SDK 10.9

    AppleAVBAudio: Version: 203.2 - SDK 10.9

    iSightAudio: Version: 7.7.3 - SDK 10.9

iTunes Plug-ins: ?

    Quartz Composer Visualizer: Version: 1.4 - SDK 10.9

3rd Party Preference Panes: ?

    Flash Player  Support

    Flip4Mac WMV  Support

    MediaLink  Support

    Rapport  Support

Time Machine: ?

    Skip System Files: NO

    Auto backup: YES

    Volumes being backed up:

        Macintosh HD: Disk size: 930.71 GB Disk used: 374.22 GB

    Destinations:

        My Book [Local] (Last used)

        Total size: 931.36 GB

        Total number of backups: 170

        Oldest backup: 2011-04-30 15:38:33 +0000

        Last backup: 2014-08-24 20:16:22 +0000

        Size of backup disk: Too small

            Backup size 931.36 GB < (Disk used 374.22 GB X 3)

    Time Machine details may not be accurate.

    All volumes being backed up may not be listed.

Top Processes by CPU: ?

         2%    WindowServer

         1%    mds_stores

         0%    fontd

         0%    AddressBookSourceSync

         0%    mds

Top Processes by Memory: ?

    254 MB    firefox

    123 MB    com.apple.IconServicesAgent

    115 MB    mds_stores

    102 MB    Finder

    90 MB    WindowServer

Virtual Memory Information: ?

    1.80 GB    Free RAM

    1.46 GB    Active RAM

    222 MB    Inactive RAM

    535 MB    Wired RAM

    417 MB    Page-ins

    0 B    Page-outs