Remington Thunderbolts may be the culprit. That is quite possibly the worst ammo money can buy. It doesn't take many rounds of that stuff to foul up the bore of a gun so bad as to leave it looking like a sewer pipe. I would imagine a pistol bore could get so thoroughly caked in lead fouling from shooting Remington Thunderbolts as to cause some sort of tumbling issues.
Cleaning out bore and sticking to plated ammo would likely solve the issue without necessarily having to resort to "high end" ammo.
"My momma, she done told me, there's heart and then there's hate
If one of 'em's inside of you, the other one, it aint."