Random musings from an owner of the previous model ...
1) Were you checking the batteries under load? A typical behavior for weak batteries is increased internal resistance which means they read decent voltage into a minuscule load like a voltmeter, but the output drops rapidly with increased current of a real load. Alas, I can't claim to have any idea what an appropriate load would be.
2) I believe the shutter system draws power while it's open (the reason for the optional T settings) and 8 seconds would be the most stressful, battery-wise. I also thought the Ai differed from previous models in that the shutter doesn't work at all without a battery, versus the earlier models which default to 1/500.
3) What finder are you using -- is it passive, or one of the metering varieties. I ask this because I seem to recall the finder can override the camera body settings. But since all my finders are non-metered, I have no first hand experience.
So I don't know if I'm even close to anything useful here. (Probably not. ) Have you used 8 seconds in the past successfully? It could be that contact on the speed switch is broken or dirty. It's a shame you don't have another lens available. I like these cameras but having substitutes for the various components sure helps with troubleshooting, not to mention provides enough redundancy to keep shooting past problems that crop up.
Good luck with it.