I had similar problems. It ended up being bad gas. The gas wasn't igniting properly and the computer attempted to correct for that by wildly adjusting the timing. There was no visible water and the gas easily lit with a match, so I ended up tearing a lot of the car apart looking for the problem. In frustration I had it towed to a shop where they dumped the whole tank and started over and it fired right up. On the up side the whole thing only cost me $500 including the tow.
http://987forum.com/forums/showthread.php?t=23121&page=4&pp=20&highlight=waterlogged
What I did might help.
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