Things are going pretty well with the project. I actually just received some MCP6021s, so I've used that in place of the LT1218s originally suggested. I've got it assembled, powered by a 9V battery and 5V regulator, and its doing pretty much what I'm expecting. I've made one other change, I've put in a 30k resistor between Vcc and the collector of the PNP that regulates the 555 timing. The reason for doing so is that my sensors are actually closer to infinite resistance at rest, not 5MOhm. This resistor let's the circuit rest at 4kHz when there's no force applied to the sensor.
However, the whole circuit is very sensitive to feedback from my computer. I've got it hooked up to my laptop audio line-ins, and there's two types of feedback I'm noticing. 1. When the laptop is plugged into the AC mains, the frequency of the 555 jumps up. 2. whenever I touch the cursor touch pad on my laptop, the frequency jumps up even more significantly. What's interesting is that, if I'm also touching the circuit ground, the effect is mostly blunted.
Not sure why this is happening, but I have a hunch that the ground potential is being lifted and hence the capacitance at C1 is decreased. Maybe that's not the case, but regardless, is there any way to better isolate the circuit from these effects?
EDIT:
the problems is less prominent with the TL082 in the circuit, so I've kept it in there for now. When the circuit is hooked to my laptop and my laptop is running on battery power, the output of the timing circuit is near perfect! It also seems pretty immune to any feedback from me touching the laptop. The problem seems to be a 60Hz inductance, because, if the laptop is on my lap, and I even so much as touch the ground of the laptop AC adapter before plugging it in, the circuit output gets very noisy. I captured a few seconds of data on from the sound card, and sure enough, looking at the waveform it seems that the frequency of the timer output is biased up and down about 60x per second.
Would an audio isolation transformer fix the problem? I know they're used to avoid ground loops, but I might just get a backward inductance of the 60Hz noise with the trasnformer, is that likely? Also, I don't have any around, and while I could probably go buy one, is there another solution to filter out the fluctuations in the ground potential? I mean, there's gotta be another way, because this problem doesn't happen why my ipod is hooked up to my laptop, and I find it hard to believe that there's an isolation transformer in my ipod.