Jimmy Buffett it's true distributors make mistakes just like everyone, however since your A.C. has been in and problem free for 5 years I'll bet being undersized is not the problem. I'm guessing you have a 5hp 2 stage with 80 gal tank. If so that's big enough for 2 SGXS with bifold
doors (Ryko
doors are air pigs) Please check the following:
1. Oil level, too much is as bad as not enough. you should be able to see the level in the sight glass
2. Check your air filter located on the pump if you've never changed it its time.
3. Make sure the belts are turning the correct way if they are spinning backwards the A.C. will still work but the cooling air is blown away from the pump instead of across it. I doubt this is it though since it's been opperating correctly for so long.
4. Still can't get it let me know, i'm in Iowa also and may be able to stop by and find the problem.