Vent lock appears to be working in reverse

I've had this happen to two consecutive batches of beer and I'm wondering if maybe I've either missed something or maybe my equipment is failing. Here's the situation:

In both cases fermentation kicked off within 6 hours and the vent lock was bubbling away at a rate of one to two bubbles per second. However, this slowly begins to peter out over the next 24-36 hours to the point it not only completely stops, but a vacuum appears to have begun in the fermenter. I say this because the water level in the vent lock starts to draw up the inside chamber. A flick of the vent lock rights things, but over the next couple hours it happens again.

In the first case, fermentation never completed. It hit 1.030 gravity within the first week and held there for two weeks, even after re-pitching with new yeast at the end of the second week, before I finally just bottled it.

In the current case, I have not yet taken a gravity reading, but the situation seems so similar to the previous batch, that I'm almost convinced I have the same problem.

I am very clean; I used Star San (1 oz to 5 gallons of water) each time, and ever bit of equipment is sanitized. The first batch was drinkable, so I'm fairly certain it's not a contamination issue. I wondered about temperature, but have taken steps to stabilize things.

Any thoughts?

the only time I have seen this happen for my own brews is when the air outside the fermeter is cooler than the brew and air inside the fermenter.

Fermentation produces heat, but when things settle down and the inside of the fermenter cools off, the CO2 in the headspace will want to shrink in volume and can create a vacuum inside the carboy.

However, if the whole system is being temperature controlled, you really shouldn't be seeing this because the beer will cool down while still producing CO2 so the system stays in balance.

Where do you have your fermenter? Is it someplace that has large temp changes during the day?

Yes, unfortunately, the room I have available at the moment is the laundry room, and given we're in the middle of fall here the room does suffer temperature variance. It's the same room the last batch was in and that was the first batch I did in that room. I knew the room would create some issues, so I followed these steps I found with some digging:

http://www.brew-dudes.com/heating-pad-for-fermentation/219

I'd hoped to mitigate the problem. Are things salvageable at this point? Should I move to a different room and keep the same rig?

You can move the thing if you want. But, really, I would not worry about a little water from the airlock getting into your beer. The beer has alcohol in it at this point and can protect itself to some degree.

Personally, I would leave it where it is, but replace the water in the airlock with vodka (or 50/50 water/vodka). That way if a little seeps in, you're really safe since it'll still be super sanitary.

Don't worry about it. It'll be fine.