2002 WRX 2.0L Manual Trans.
In cold weather the car will buck and shake once in gear(most prevalent in gears 1-3) in lower-mid RPMs (2800-3800). In warm weather it's perfectly fine. I warm the car up for at least 5 minutes everyday (to the point where the temp gauge is up and the idle is at a normal 500-1000 RPM) but it still has the problem. I make sure to be gentle driving while it's still warming up, but it doesn't seem to help.
Recently the check engine light has been blinking randomly until the car gets warm and then disappears, thus making it difficult for mechanics to diagnose the problem when I take the car in. The shudder is once the car is already fully engaged, so it's not me shifting poorly. However, it might still be the clutch slipping. I am no mechanic though so it could be the clutch, fuel mix, electrical, etc. I'd like to get people's opinions.