'93 Honda Prelude Si A/C Problems

Hi:

I've been experiencing some trouble with my A/C. I have a '93 Honda Prelude Si (has 280,000, 5MT, 2whl dr).

Normally my A/C works great. But, at the beginning of the warm season I noticed it was cooling as efficiently as before. I assumed (because of it's age) it probably needed a shot of r134a. I had the system evacuated and recharged. However, I am having trouble with the A/C blowing cold, then hot; cold, then hot.

First fix: mechanic suggested the compressor was shutting off prematurely and was caused by a thermal pressure switch located on the compressor. That was replaced and I'm still experiencing the same problem.

Second fix: mechanic suggested changing the expansion valve as he said the high pressure side was shutting off the compressor at around 100 psi instead of at the limit of 200 psi. So, the expansion valve was changed. *Note: I couldn't find a receiver/drier to fit that day so, he put my old receiver/drier back on.*
The mechanic also said he thought there was too much oil in the system and took some out (?). He also said that he was putting dye in the system to see if there were any leaks.

I then had a problem with a leaking oil cooler hose that was causing my car to overheat. I had the two hoses replaced and noticed a difference in the performance of the A/C for about 2 weeks (don't know what the relation is).

Now, I'm starting to have the same problem again. Cold air, then hot air, then cold air then hot air.

I don't know if this helps any but, I have noticed that my car does seem to heat up more quickly in the morning (the hoopty is usually fairly cold natured and takes a minute or two to get going) When the A/C was working correctly the past two weeks I noticed the fans seemed to never shut off and it seemed to be causing a constant load on the engine. Now that the A/C is back to the cold/hot action the engine seems to be performing as usual.

My mechanic suggests changing the evaporator.

Could you give me some suggestions and perhaps an explanation as to why he would think the evaporator is bad?

Thanks for your advice!