i have been through 300 round of prohunter madness, its now all sorted.
brief history as follows (you can view my previous posts for more details).
got prohunter mountain .243 with T8 and S&B 8*56 july 2006.
my first centrefire.
started out ok, but roaming groups, always sub 1".
the some big jumps from zero (about 4") but still grouping.
put 60 shots through without cleaning from new
fitted second bipod stud, further back to reduce flex. still use that one now.
gun went back to sportsman, had absorber pad removed (its under the barrell at front).
it also had 3" copper fused to the rifling, sportsman fully de coppered.
when returned it was shooting excellently, but after time it started wandering again. i was using boresnake.
i did used to leave it in a hot car whilst at work, and use it at lunchtime, this may have effected it.
i then filled the front end with acraglass, but havent bedded the action. there is still flex but it doesnt touch the barrel.
its now shooting cock on, sub 1" all day long ( apart from when it gets hot!)
i'm not sure what actually caused it, but what i would advise to you is to check all of the obvious, and then make sure you cleaning regime is sorted. my current cleaning regime is ... never clean it unless absolutely necessary. if you clean it, fully de copper it (takes 2 nights). never use a boresnake. first shot out from clean flies 1" out, then its cock on all the way. if you dont do a full de coppper- it takes about 10 shots to settle down. stay away from the boresnake. i havent cleaned it for weeks.
sportsman were very good and even said they would change it for something else if i wasnt happy. Brian down there did the floating for me.
hope this is of some use.