Does anyone know how many times the NES game "Gradius" ramps up in difficulty?

I know that this game becomes harder on the second playthrough (just like a lot of other NES games, like Super Mario Bros, Contra, Castlevania etc), but how many times does it keep doing this?
I played through this game a while ago and I noticed a few interesting although fairly simple things on the second playthrough, like enemies being faster and shooting more projectiles etc.
These kinds of things could in theory keep ramping up forever until the game could no longer handle it, so when exactly does it stop?

Don't know about Gradius, but I know that Ghost N Goblins increases in difficulty until the game is impossible to complete, so that may follow the same format ??

I think Ghost N Goblins becomes impossible on its seventh or sixth straight playthrough.

I guess they might ramp up indefinitely, at least until the game cannot handle the intensity anymore. :p

Anyway, I made it to stage 6 in Gradius a while ago (the one with all the flying star enemies or whatever, and all the blue crap everywhere) on the second round, and that stage gets pretty ridiculous at that point.
Basically, if you die there then you might as well reset the game, since the enemies will be way too fast and chase after you all over the stage and surround you, and you won't be able to fly away or defend yourself properly.