It does sound like the deraillier needs adjustment especially since you have changed the spring. The change will mean there is a different tension in the deraillier and therefore it will be the top & bottom ratios that will cause problems...
I think the thing to check next is the trim screws to the back of the deraillier casing. One will trim top (wheel side of the cassette) and the other bottom (drop-out side of the cassette). Hopefully judicious adjustment should solve the problem..
I like compliments... flattery will get you everywhere
Try this. The high gear is the small ring, if you look at it in that gear count the teeth the chain is using. If you put too much spring in the twist portion of the derailleur it will rock it back and take the chain off some of the teeth. It is a fine balance to get the adjustments correct. Have fun, Dave
PS: If the edges of the teeth are rounded over nothing will fix it. You will have to get a new ring.
Be gentle on your adjustments - too far and you'll never get the thing back working properly - I did that the first time I tried it and I could never get the indexing back on that bike however I tried..The best way is put the bike upside down and slowly pedal the bike... you'll be able to eyeball the adjustments that way..
Thanks for the page comment - I havent put anything up on there for ages.. alot of things going on lol..
It is best to do the shifting adjustments with the bike right side up. Up side down will get it close but not on the money. Before I bought a bike service stand I would suspend the rear wheel from a rope over a garage rafter, then do the adjustment never turning more than an 1/8 turn at a time.