Have you ran a full scan with Malwarebytes? It's a free program that will hunt down hard to find malware on your system.
Also, AVG & Windows 7 doesn't make for a happy marriage. There may be leftover traces of the program on your computer. It's one of the most stubborn AV's to remove, especially the IS suite.
If you've ran all of the tests that the other members suggested, and all of your hardware (including RAM, HDD & MB) are good, a reinstall may be in order. I had to reinstall mine when new, because Windows Backup wouldn't complete (I no longer use that program).
If you decide to do this, make sure beforehand that you have the most current drivers for your system, for all devices, as well as the iso or exe files to re-install your programs. And when you do reinstall, at first, just install what you need, one at a time. That way, if a particular program is the problem, the issue should duplicate itself, and you can just uninstall the program.
Finally, whatever repair was done, is there a warranty on it? Most reputable shops stands behind what they do for a minimum of 90 days. And if the computer is still under warranty, call the OEM & report the issue.
Best of Luck,
Cat