Furnace produces explosions?

Hello all,

I've got a problem with my furnace, and I hope this fine community might be able to help. Actually, it's not my furnace; I'm renting the lower half of a duplex, and the offending furnace actually belongs to the upper half, who are also renting.

The problem is this: every once in a while, the furnace produces a loud bang, and by "loud" I mean floor-shaking. And by "every once in a while" I mean one to two times a day. This was happening all of last winter and spring. The furnace hasn't been turned on yet this season, and I'm genuinely concerned for my safety.

Yes, I've read about delayed ignition, and I'm sure that's what's going on here. However, here's some more information that doesn't seem to add up:

- The furnace doesn't produce the explosion on every start (it's only 1-2 times a day)

- The furnace was examined by technicians on two occasions. Both times, it was cleaned thoroughly (according to my upstairs neighbor; I wasn't there), but it continued to produce the bang with the same frequency. The second technician supposedly said that this behavior is expected in an aging furnace (Of course I don't believe this for a second). The landlord, who lives in a different house, refuses to replace the furnace, or have any more technicians examine it.

For your viewing pleasure, I captured one of the explosions using a webcam (this was last spring):

www.youtube.com/watch?v=cnNv6evW9vw

My theory so far is this: When the furnace shuts off after a successful run, the gas valve closes down, but not all the way, allowing a trickle of gas to flow through. This gas gradually fills up the chamber, and the next time the furnace starts, it ignites the gas explosively. Does this sound plausible?

Does anyone have an idea of what else could be repaired / cleaned / replaced in this furnace?

P.S. It's a direct spark ignition furnace (Amana Air Command, probably from early 1980s).