All the scans came up clean and Autoruns looks OK for the most part (I will add my personal comments at the end) so I don't believe your system has a malicious infection at this time. I believe it has to do more with either a problem with the operating system or more so a Hardware issue since you made this statement in your first post:

There was nothing wrong before I shut it down a week ago, then the problem started immediately happening when I turned it on again.
Possibly an electric spike/surge could have made some damage with the mobo or even the PSU enough to cause abnormalities even though the damage part still runs at least for a certain time before acting up.

Then again, the OS might have been damaged due a nasty jolt the HD had taken...of course these are all valid possibilities but I can't say for certain with this much info and not much troubleshooting.

On a more practical and logical approach, I would look into the services/process running in the background. You mentioned nothing over 30% or no CPU hogs which is fine but this still wouldn't mean everything is peachy.

Also occasional slowness is with the entire system or just network/Internet based programs? That could help you isolate the culprit easier.

On the hardware side, when the slowness occurs, make sure all the fans are working (the CPU heatsink fan/ PSU fan in the back), no excessive heat or also sounds coming from the case. Possibly damaged or faulty components: HardDrive, RAM and/or PSU. If overheating is the reason, the problems you experience should occur after having the system on for similar amount of time but if it is due to a faulty component, it'd be hard to find a pattern.

The below is my rundown on the autoruns log but like I said nothing that could be the cause as far as I can see (I skipped the entries that are unnecessary since this purpose of this log was to spot possible invalid entries):

Pretty common, I would at least disable it by unchecking the box, or delete it either action is fine, your call:
HKCU\SOFTWARE\Microsoft\Internet Explorer\Desktop\Components
+ 0 File not found: About:Home
Pretty common, I would at least disable it by unchecking the box, or delete it either action is fine, your call:
HKLM\Software\Microsoft\Windows\CurrentVersion\She ll Extensions\Approved
+ Display Panning CPL Extension File not found: deskpan.dll
I'd suggest deleting this one:
Task Scheduler
+ MP Scheduled Quick Scan.job File not found: Files\Microsoft
Again, unlikely that any of the above was the cause but better to clean them up.