When I have the a/c on the rps fluctuate between regular idle and about 850-900. Although I expect this, it seems that its doing it too often. Every 30 seconds or so. Is this normal? Also a side note, if I were to get a thermostat what should I go with?

 

if the A/C clutch is engaging every 30 seconds you probably are low on R134A Refrigerant

 

I had a similar problem. Whenever I turned on the A/C or defrost (which turns on the A/C) the rpms would jump up and down at stop lights and blow warm air. I thought it was my IAC so I got a new one and that didnt fix it. I then got on mustangforums and found that it was the A/C recycling switch. Its about $13. Hope this helps.

 

Thanks a lot ill look into both. I haven't really put any refrigerant since I bought the car so that is a huge possibility.

Also, after turning the a/c on that cutting it off, I notice that my rpms would hang, but in the way that perhaps my iac is messed up (this will be iac #3). It seened to have activated my iac getting stuck anyone experience something similar?

 

What this member said. Its trying to build pressure and cant.
Id check for a leak also with a color dye refrigerant.

 

i would just try charging it first and go from there. most likley its a very small leak that may take a year to notice.