Hi,
1.
Which other games are causing this issue?
2.
Were you able to play these games without any issues earlier?
3.
Did you make any recent hardware or software changes on your computer prior to this issue?
Step 1: Try running the
Portal 2 game in
compatibility mode
and check.
Step 2: Check for error messages in event viewer. If you find any error messages then post back the exact error message so that we can
assist you better.
Steps to check Application Event log:
a. Click on start.
b. In the search box type eventvwr.msc and press enter.
c. Now click on Application under Event Viewer (local).
d. Now look for event log on right hand side of the Event Viewer windows.
http://windows.microsoft.com/en-US/windows-vista/Open-Event-Viewer
http://windows.microsoft.com/en-US/windows-vista/What-information-appears-in-event-logs-Event-Viewer
Also refer:
Fixing game performance problems
http://windows.microsoft.com/en-US/windows7/Fixing-game-performance-problems