Mystery problem on 1993 Honda Accord (longish)
 

I'm having this intermittent problem on my 1993 Honda Accord automatic and can't diagnose it.
My car had been running fine,but about two months ago after a rain, the car began to hesitate.
I decided to have it towed to my mechanic. He resoldered the main relay and put in a new igniter. Car ran fine for a couple months until the next rainstorm.
Car ran fine in the morning after the rain. In the afternoon,the car had the same problem as before, but this time I heard a lot of random clicking at the gearshift. The type of clicking you hear when you step on the brake and release. Except I didn't have my foot on the brake.
At the same time I could hear a kind of a buzzing sound coming from the distributor,the D4 light was flickering,going to park then going back to D4.
While the car was in D4, it started out in second gear.
Took to another mechanic and he put a new main relay in. Had a transmission mechanic test drive, but he thought it was an electrical problem.
My mechanic checked the neutral safety switch and the TCU and they were good. He test drove it for about a half hour and he said the car was fine. I drove it again for a couple days same thing happened as before.
But this time I heard a rattling under the car and when I stepped on the gas, it didn't move for a few seconds and then it started to accelerate like a tortoise. Hardly any power.
So now it's back at the mechanic again and he's checking the catalytic converter and the fuel pump, fuses, etc. He's had it for a couple days and test driving it but he said the car is running fine. He suspects something in the ignition system
He said he can't diagnose the problem until it acts up again. So does anybody have any idea what may be going on? This car only has 85,000 miles and hoping its an easy fix.

Honestly I would start by trying to figure out where the water issue is, assuming there is one. Has the windshield ever been replaced? Are the drain holes under the hood near the hood hinges free of debris so that water drains properly? Are you running the AC on the rainy days and is the AC drain line free or is it clogged which will put water into the car near the computers. Often times I will run a garden hose over the car, get inside with a flash light and look for the leak. If there is a leak into the cabin area the location of the leak can then help with the diagnostics of an electrical malfunction.

That's a good point about checking for a possible water leak.
The thing is the car was hesitating and stalling about two weeks after the rain,unless there some areas that hadn't dried out

It is not necessarily that they did not dry out, it could be corrosion. Thus if a water leak is this issue that is a must solve otherwise if the electrical problem(s) is fixed it is likely to reoccur if the root of the problem is not fixed.

I understand.
I guess the mystery is why it is intermittent as opposed to constant.
It acted up last week one month after the rain.
Thanks for your input.