I have never had any trouble with a leg of lamb and I buy mine where they are cheapest, which often means Aldi.
If it is frozen: defreeze completely.
Slice as much peeled garlic as you like (if you like it at all) and place each slice into a little slit that you make with a small sharp knife on the surface of the joint that will be upwards in the oven, Spread thyme over the surface and salt and pepper.
Place in a roasting tin with a little water in it.
Put it on the second lowest tier of the oven.
The oven should be cold - this is important.
Turn on the oven, setting it to 175 degrees centigrade and 2 hours cooking time.
Then go off and do anything you want for the next two hours.
If you want roast potatoes with it, peel them now, put them in a well-greased dish, add a little water, salt them and pop them in the oven after the first half-hour has passed.
In an electric oven you can stand them on the bottom; if you are cooking in a gas oven they will need to be in the lowest tier.
I have never had the problems you describe when I have used this method. Even more surprisingly - it works well for my adult son as well!
That says something, as neither DH nor I could interest him in cookery when he lived at home - he has learned in self-defence after finding a wife who neither can nor will cook!
No, I am not having a dig at her, I am still laughing about DS realising he should have listened to us years ago and learned to cook! (I had succeeded in teaching him to sort washing in colours. Just as well, as guess who does all their laundry. You've got it; DS does!)