I have a 1999 Honda Civic EX Automatic Transmission Bought at 138k miles currently 199k miles. This issue has been going on since the first Summer I owned the car, and I'm pretty sure it's why the old owner sold it.

When its over 90F outside, the car bogs down hard. Whether or not the AC is on seems to make no difference. Acceleration is wonky and switches between strong and week every few seconds while accelerating.
When the AC is on, it's constantly clicking on and off. I have had it checked at 2 shops and both said all parts of the AC system are fine and the freon levels are good.
The car will stall at red lights, but not actually die. RPMs drop to 0-100 and the engine shakes so hard you can see the car shaking from outside.
When its under 90F (and I literally mean 89F and below) the car runs perfectly fine with no issues.

I have changed every sensor Google has suggested for this issue and made sure the car is not overheating. The head gasket shows no signs of being bad and I put a brand new Catalytic Converter on it 2 years ago.
The radiator was changed 2 years ago. The thermostat was changed last year. The engine oil levels are good and oil changes have been on schedule.
The entire air intake system has been replaced including connected sensors. The spark plugs were replaced 3 years ago and last year, o2 sensors replaced, distributor (and its cap) replaced, spark plug wires replaced.
Tried changing the IAC, but that made it stall out and actually die. Switched back to old IAC after that.

I have found a 20 year old post on these forums that seems similar. In the end they ended up chipping the ECU to disable the knock sensor. That is WAY beyond my skill set and I would rather not have to go that far.
Also their issue didn't mention the temp outside, and stated the problems don't start until the engine is up to temp. Not sure if it's the same issue.

Now that it's Summer, I can begin testing theories again. Getting a multi meter and checking my Ignition coils tomorrow (idea given by another post). Any other Ideas would be helpful. Thanks!

 

any check engine light codes?
when the engine is acting up, check the timing. see if its too retarded.
clean the IAC with a wire brush and carb cleaner

 

No engine light/codes.
A mechanic has checked the timing with a timing light and said it was fine.
Before reattaching the IAC, It was thoroughly cleaned out.

 

Mine is doing this. While driving a short trip, timing advance can be retarded into negative numbers. Other times it is good. Mechanical timing is good. Ignition timing at idle is 12°, and solid. One of my last possible issues is the knock sensor, which I have (OEM) in hand, but have not been able to take the time to do it. Dang wiring harness is right in the way.

 

What IAC do you have. google it and post a picture please