Q: Imac Extreme Lagging

Hello to all,

I run an iMac (specified below), updated to Yosemite a couple of weeks ago. A couple of days ago the mac started lagging to the point I can't work; cursor lags, typing lags, opening apps bounce on the dock for a while, closing them takes for ever, mail is slowest ever, etc... I've tried resetting PRAM, I ran utility disk (revised and repaired), I reset SMC, I've tried working single apps, but nothing makes a difference; it keeps lagging and activity monitor shows steady system processes but spiking user processes. In case its worth mentioning, I use Google Drive, iCloud Drive, and Dropbox (its just the way I've devided my folders to keep low volumes). My internet service is slow and unstable (around 2mbps; its all that is available at my location). No apple stores or Genius Bars at reach. Any suggestions? Thanks in advance.

iMac (20-inch, Mid 2007)

2 GHz Intel Core 2 Duo

4 GB 667 MHz DDR2 SDRAM

ATI Radeon HD 2400 XT 128 MB

Yosemite 10.10 (14A389)