1.- Try changing the OVP relay (Overvoltage Protection Relay) it is behind the battery (remove plastic cover), it is the silver one with the red top and the fuse on it. This relay has been known to create havoc on these vehicles when it goes bad.
2.- I was having the same problems with the climate control unit. I took it out and resoldered all the contacts related to the pushbuttons, as of today it still works like a charm.
3.- The cruise control is another problem with these vehicles. The amplifier, or the actuator may go bad. Since yours works at times, I imagine it can be fixed. Use the search in the forum and inform yourself before you spend any more money. I have saved hundreds, if not thousands by reading in this forum.
Right know I am going thru some problems with my own vehicle. I am going thru all the suggestions found here trying to solve it before bringing it to a mechanic.
I am suffering from high-oscillating idle (600-1000 RPM or fixed at 1200 sometimes) when in park or neutral. I removed and cleaned the Idle Control Valve twice (consecutive weekends), and still the problem occurs. I also cleaned the air intake and surrounded areas. The response is much better during acceleration.
I also checked for vacuum leaks with starter spray, and found none.
Believe me, you are in the right place if you enjoy doing some work in your car.
Search, read and work on your car.
By the way,
Welcome to the forum.
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car