I have wireless lavs, Sennheiser ew100 G2's to be precise.

Sometimes I get interference and for the love of God cannot figure out why. I rescan and the new frequencies dont solve it. I'm also sometimes in the middle of nowhere so there shouldn't be anything around causing problems. But I'm just getting a tad of background noise, some fuzzy sounding stuff, and I hate it.

If that happens.....what should the first things I look at be? What are the most likely culprits?

Loose connection at the mic or antenna or low batterys. Could be something making the cable from the mic rattle around. Hard to say for sure. I hate when stuff like that happens.

When you scan are you scanning a 'bank' the G2 system has 8 banks (1-8)?
For example if the RX is set to bank 2 and you do a scan..... it scans ONLY bank 2, in each of the 8 banks there are approx 4-6 inter-mod free frequencies. (so really you are only scanning 4-6 frequencies and not the full available spectrum)
Its often better to do a scan in 'U' bank which is the user bank and actually has a wider view on available frequencies. The problem by doing it this on 'U' Bank is that you may get intermodulating frequencies. And if you do select the second ,third, fourth mic etc. well away from each other when selecting frequencies.

Nowadays you need to determine if the interference is from non-radio mic signals - the new phone networks are the usual suspect, or if it's on-frequency interference from other radio mics, or if it's intermodulation from radio mics on nearby channels. Personally, scanning for interference free channels is rather pointless, because at best, it means the channel they suggest is free - but could fail shortly afterwards when another user moves, or comes into range. If you are working just one system, then a cheap wide band scanner is a great tool as you can listen and find real empty channels - back off the squelch and you may hear faint radio mics in use, when your proper receiver doesn't.

The different types of interference have characteristics, so whenever I hear noises, I'm pretty good at putting them into categories, and taking appropriate action.

It is not apparent from your stated symptoms that you problem really is "interference" it sounds more like simply poor performance from inexpensive gear, and perhaps deteriorated performance from age, wear-n-tear or trauma.

You making sure all cell phones are off (not on mute) in the area of the shoot? Just askin'.

Can you post a clip. It's easier to specifically ID the issue if we can hear it.