air-conditioner repair

I appreciate any insight/recommendations you can provide in trouble shooting an air-conditioning problem I have with my 1986 300SDL with 90K miles, original owner.
Last week I drove my car for about 2 hours (AC worked beautifully) then let it rest for about 3 hours- went back into the car and the air-conditioning would not kick in. It worked fine for the initial 2 hour journey.
Even though the cold air did not come on- The fan blowed like heck. While I was driving and sweating I played with the on off controls, windshield defogger and after doing this for about 20 minutes the air conditioner went on and stayed on.
This week I used the car and the air-conditioner worked fine again for about 3 hours. When I shut off the car and returned with 30 minutes, the AC would not go on at all even after doing the same mickey mouse on/off stuff with the climate controls.
I also happened to notice simultaneously that the tachometer which usually works is also not working. Is it possible that these 2 systems can be connected and will give you a clue to the problem?
It would sincerely be appreciate if you could help me diagnose the problem or kindly forward this email to someone that might be able to help.
\Thank you for your time and expertise.
Note I did check the fuses and relays to make sure they were clean.
Steve
sdagz@aol.com