Off the top of my head I’d say...... you need better lifters.

But, I do have a question......
Lifter bores are dry on the Indy block unless you get the lifter oiling option........ does your block have that option?

If you’re running dry lifter bores, you’re not adequately oiling the lifters.

IMO, the Sportsman lifters are going to be marginal with 900lbs open load.

If the lifter bores are dry I’d talk to Isky and see what they recommend for that application.

From what I’ve read, the H-11 style springs don’t seem to have the high rpm stability that some of the more modern stuff does.

At 7K........ you could probably run less open load too.
What’s the lift?
Is the cam anything weird?

Also, if it’s a std core size cam........ with that spring open load, the cam could be flexing, adding another variable to the valvetrain tuning...... which could be causing some strange things to occur.