First thing I would do is clean the bore, I mean REALLY clean the bore. This is not a flame toward you. I have many rifles come in every year that are supposedly clean but after a week in the shop and getting several pennies worth of copper out of the bore, they really start to shoot again.
I do not recommend the use of Sweets 7.62 or Barnes CR-10 all the time but if you have a rifle such as a 308 where accuracy goes south I do soak the bore with these solvents to make sure the bore is clean. Repeat using them until the push out of the muzzle as clear and clean as they went into the bore.
In a barrel like this, its not uncommon to see a copper build up just ahead of the throat. This is what can cause accuracy to drop off as the bullets are swaged down in this tighter area of bore but then when the bullet passes this area, the bullets fit to the bore is loose and accuracy drops off because there is not enough pressure in the bore to bump up the bullets base to fill the bore again.
I would also recommend scoping the bore just to make sure its totally clean. If the accuracy does not come back, they you may want to have the muzzle recrowned, its cheap and can make a HUGE difference in accuracy is there has been any crown damage, even slight crown damage will destroy accuracy. Often it does not even need to be bad enough to see and still it can cut your accuracy.
I recommend this again because its cheap, should be less then $25-$30 to recrown your muzzle by most smiths.
If that does not bring the accuracy back, then yes it may be time to rebarrel.
Another thing to look at is if the bolt lugs are contacting the receiver evenly. That usually shows up as vertical stringing as pressures increase but as your barrel was shooting well at first but then went south, that probably is not the problem.
A 308 should get you 2000 rounds of accuracy life pretty easily, if it was shooting well before 200 rounds and went south after that, I would say there is something else going on other then a bad bore, that is unless you shot the first 100 rounds all at once as fast as you could!!!