You dont need a radiator shop, fixing a coolant issue is so easy. at this point you could have thrown a brand new coolant system on the thing for $200... damn shops these days... its like legal rape. lol.
It sounds like you have a leak, check the coolant flang on the back of the head. Also check the coolant expansion tank for cracks or leakage. Check the cap as well, if its chipped cracked or just brittle it could cause that issue. For shts and giggles i would go spend the $6 on a new cap and see if that helps. i wouldn't be surprised if that's the issue. It doesn't have to be a huge leak, just enough to loose pressure.