3rd Gear Pops Out

[ICEBERG1]

So I picked up a 2008 LGT it has 32k miles on it. I don't drive very aggressively and have owned a lot of manual cars. I feel I am very proficient at appropriately operating a manual tranny. The problem I have been noticing is that occasionally 3rd gear will pop out. This usually happens when I am cruising along around 2-2.5k rpms and hit a bump or suddenly let off the gas pedal. When in 3rd gear the shifter seems to move excessively when accelerating or decelaerating compared to the other gears. While the car is parked with clutch pressed in I have moved the shifter through the different gears and all gears with the exception of 3rd lock in really solid. 3rd, however, seems to be really loose and with very lite pressure applied to shifter it will pop out of gear. i used to have an 06 LGT and never once experienced anything like this. As this is a used car I picked up I cannot account for how it may have been treated previously and of course durring test drive it did not exhibit this issue. I looked through the forum and only found a similar issue associated with reverse gear and that sounded like it envolved quick acceleration. I am wondering if any of you have any insight as to what could be going on. I am about to take it to a dealer or local repair shop but thought I'd throw the question out here first. Thanks!