I am working on a friend's '91 Camry that runs very well at times, but after it has rained or just when it's very humid out, it runs very rough. It will start, but when you give it gas it coughs/sputters and finally usually stalls and is hard to start. It will sputter more if you give it gas quickly but if you slowly let it speed up it seems to run smoothly at high RPM, so I don't think it is a bad fuel pump, although I can't be sure.
It looks like it has the original spark plug wires and I wonder if they could be degraded and maybe moisture causes them to short out, or if there is something in the distributor that could be sensitive to moisture.
Also, I would think if it was a moisture problem that it would run better after a few minutes and the engine had heated up and evaporated any moisture, but apparently that doesn't happen; the owner said she drove it to the store and it only started stalling after she had gotten there and turned it off, then when she started it again it sputtered and finally stalled.
I have a fuel pressure gauge, but there is no easy way to connect it - no Shraeder valve - on the fuel line, so I can't monitor the fuel pressure.
If anyone has had a similar problem please let me know. Any help is greatly appreciated.
Charlie

 

It really sounds like you are getting moisture under your distributor cap, I would take it off and look if it is wet under there, or has any signs of carbon tracking. If so I would replace the cap (as it comes with a new gasket) to keep the moisture out.