can pH strips go bad? / 5.2 stabilizer

There's no way your mash was 3.8 unless you were using highly acidified water to begin with. But you can measure your water pH to be sure. Most water starts alkali and if anything you have to work to bring down the mash pH.

The pH mostly affects enzyme activity, and so mash efficiency. There is some minor affect on body, but this is small compared to the affect of mash temperature.

pH papers will only give you a ball-park reading, at best +-0.3 pH and they cannot be read under fluorescent lighting, and can be difficult to read in good lighting conditions. If you want to be sure about pH get one of the waterproof pH meters on ebay, or alternatively buy some pH calibration solution and use that with pH papers to become proficient and judging color and checking their accuracy.

You can brew well within style guidelines without worrying about pH. The grain bill, hops schedule, yeast choice is by far the bigger influence. pH primarily affects efficiency, so once you understand your typical efficiency for different types of beer, then you can adjust the recipe accordingly and not worry so much about the exact pH. But for fine tuning a beer, pH can be a useful tool once the rest is in place. A lower pH 4.9-5.1 produces a drier beer compared to a higher pH of 5.4-5.7, but again temperature, and mash thickness are much bigger influences.

A pH of 3.8 seems very unlikely. If you get a 'rogue' reading always take a new measurment. Also bear in mind that the sample must be taken at room temperature. A higher temperature will give an apparent lower pH.

I avoid the 5.2 stabilizer - some phosphoric or lactic acid can be used to reduce pH. If you need to raise pH, baking soda or chalk can be used. But again, anywhere in the 5.1-5.5 range is fine.