Unexpected power behavior (sleep, shutdowns) after repair

Question

I recently sent my MacBook Pro (Early 2008, 15") into a computer shop to have the right speaker replaced. Shortly afterwards, the machine began exhibiting unusual behavior; it started unexpectedly sleeping or shutting down while I was using it.
(Note: My MBP battery is perma-drained almost completely, so prior to sending the computer in, the computer might unexpectedly shut down if running solely on battery.)
I reset the SMC and thought nothing of it, until the mac shut down unexpectedly again, almost 24 hours exactly after the SMC was reset. I then decided to run the Apple Hardware Test.
It froze on 'testing logic board'. Twice.
I have since reset the SMC again, and am unsure if problems will continue. If these power issues do recur, what should I do?
Notes:
• I installed smcFanControl after the first shutdown. Whatever's going on, it doesn't appear to be the result of an overheat.
• Before you ask, I am not under an Apple warranty.

Answer 1

The problems have continued, albeit very intermittently. I was able to get the system log after one unexpected sleep. Here's what it said:
Jul  9 16:25:24 Dragonfly loginwindow[35]: loginwindow SleepWakeCallback WILL sleep
Jul  9 16:27:24 Dragonfly loginwindow[35]: loginwindow SleepWakeCallback will power on, Currenttime:7/9/2012 4:27:24.010 PM - Waketime:7/9/2012 4:27:24.000 PM = Deltatime:0.009685874
Jul  9 16:27:27 Dragonfly configd[13]: Sleep: Success - AC 98 - Low Power Sleep
Jul  9 16:27:27 Dragonfly configd[13]: Wake: Success - AC 98 - UHC3
Jul  9 16:27:27 Dragonfly configd[13]: Hibernate Statistics
Jul  9 16:27:28 Dragonfly configd[13]: PMConnection AirPort configd plug-in com.apple.powermanagement.applicationresponse.slowresponse 4263 ms
Jul  9 16:27:28 Dragonfly configd[13]: PMConnection IPConfiguration com.apple.powermanagement.applicationresponse.slowresponse 4270 ms
Jul  9 16:27:28 Dragonfly configd[13]: network configuration changed.
"Low Power Sleep"? On AC power, with the battery fully charged? I suspect something's up with the power management system, but I don't know enough to be sure either way. Anyone have any ideas?