Chain either won't change or slips off.... Help

pinpinipnip
pinpinipnip Posts: 2
edited March 2008 in Workshop
I have a shimano 105 compact chainring the chain was poping off (onto the crank) some times when going down hill in highest gear.
I adjusted the outerlimit screw and have somehow managed to balls it up.
It now either won't go up from the small ring to the large or falls off.
Even doing the tiniest of limit screw adjustment till it does shift up it will then falls off on to the crank.
there seems to be no happy middle ground.
It used to work. please any help before i head to the bike shop.