I am not impressed with the lock washers on my FEL frame mounting points and they seem cheaply made. I still have to check/re-tighten the bolts on a regular basis and have ruined a couple lock washers (spread them out) by trying to torque to mid point of specifications, even though I am over 100 hours. I have purchased grade 8 replacement lock washers for the next time I go through this, which will be within the next couple weeks.
However, my FEL is used for more than it is designed for by digging out lots of very large rocks and stumps. Plus I lost a C-clip for one of the FEL QA 1.18" pins at around 50 hours. But the pin has not moved a bit from its normal position, so all is well. I have not been able to find a replacement C-clip on-line, so have added it to my dealer list of parts to purchase.
I've done a fair amount of digging with mine, too, but probably not to the level you describe. I continue to check the bolts every so often but for the last 20+ hours nothing has loosen up. Given the history of the first 30 hours, I'm still not totally trusting. I'll be interested to hear if a higher quality lock washer keep things tight.