I finally got A/C hooked up on my v8 swap and put the doors/top on for the 100 degree Houston heat. Went for a test drive yesterday and found that randomly on the highway my A/C vents would not blow and cold air would only come out of the defroster. This suggests it's losing vacuum. This only lasts about 5-10 seconds, then it starts blowing out of the vents again. Any way to remedy this strange issue?