Restricting grazing for sheep is a very bad idea, - they are ruminants and need a constant throughput of feed, cutting them down can stress them and make them ill irregardless of body condition. The way to address the condition is to lower the ME & Protien of the diet, IE no supliments and longer grazing (Longer grass has a lower ME/protien and energy content and takes longer to digest than short grass, quite asside from the fact its not good for the ground to be grazed too short.
If the sheep is persistently lame - the kindest and correct thing to do is to send her off - She could likely have a joint issue, or maybe have damaged some cartilage running, or have had a foot infection in the winter, which has damaged the joint, which she will not likely recover from. Spending money treating something like that is pointless as you will spend 5x the sheeps value and are highly unlikely to see a recovery, if as you say it keeps re-occuring.
Laminitis does not really occur in sheep, and if it did, the kindest thing would be to send it off - sheep, due to the way they are build and eat are not good at recovering from illness - and As I have found, a persistently lame sheep will always be so with any treatment only really being a sticking plaster to the underlying issue. (I have fought to keep a £700 texel stud ewe good for years but ate her in the end) - make her comfortable, try anti inflammatory and if after 4 weeks no good... time to send.