P1506 and P0308 codes

Hi guys! I just got my Mustang GT 2000 back from the shop for an intake replacement (the old one was cracked and was leaking coolant, causing the cylinder 1 to misfire). They installed a new intake and it now seems to be running fine.

However, the car is now throwing a P1506 code (manufacturer controlled vehicle speed, idle control and auxiliary inputs). What could that be? The IAC valve is almost new, I installed it about 6 weeks ago.

I also have a P0308 code (cylinder 8 misfire detected), which is funny cause the main reason why I took it to the shop it's because the cylinder 1 was misfiring.

I'm gonna take it back to the shop tomorrow, but in the meantime I'd like to know if any of you has any ideas.

Thanks!
P.S. Sorry for my bad english

Yes, it does idle funny. It's around 1200-1400rpm when I start it and then goes down when I turn the AC on, to around 1000rpm.

Also, it kind of surges on its own when I'm driving, as if I had tapped the accelerator.

I took it back to the shop this morning and the guy said it was probably the IAC valve, but it's new, so I'm also inclined to think there's a leak somewhere.

As for the misfire, he also said it might be the coil.