Well, the TSB solved the problem. Thanks again to Psychos2 for bringing it to my attention.

A word of warning to anyone who attempts to replace their timing belt by themselves... I don't know if this problem was resolved in the 2000-2001 models, but apparently the Timing Belt Replacement procedure outlined in the Isuzu Shop Manual can create this problem. While the manual states that you should remove the timing belt and rotate the cam shafts and crank independently, the TSB explicitly states that this SHOULD NOT be done. The end result is having to remove the heads and realign the gears to reindex the cam shafts. Neither the mechanic nor the Isuzu technician he spoke to could really explain why this happens, but the bottom line is if you are going to do this yourself, the marks should all be lined up by rotating the crank BEFORE the belt is removed, and NOT rotated again until the new belt is back on and tensioned. I know some others have had success doing this themselves using the manual procedures, but I know I followed the directions correctly and it wound up costing me several days of my time, the cost of the parts, plus over $300 to have this mess fixed. You've been warned!