I have a Hitachi NR90GC cordless framing nailer. The manual says the nails should be paper collated strip clipped-head nails 2'' to 3 1/2'',.113 to .131. It does not specifiy a degree for the nails, and there are many on the market. Thanks in advance. ****

Maybe there can be a problem with a rip in the cilinder its a very thin cilinder and its neccesary to replace with all the parts on it because of the extreme heat build up.
cilinder assy 42 41A possible necessary, but first inspect the cilinder carefully on a rift. some other rings or parts broken or worn.
Hitachi NR90GC 3 1 2 Gas Powered Clipped Head Framing Nailer Parts

Could be bad gas canister or needs cleaning. How many boxes of nails You shot thru that tool?

The manual for your gun specifies 34-degree nails from 2 to 3 1/2 inches long. 28-degree, wire collated nails are a Stanley-Bostitch thing and they only work in their nail guns, they will not work in your nail gun. You need 34-degree, paper-tape collated nails. I know Senco framing nails will work in your gun and I think Duo-fast makes 34-drgree nails but check that before buying them.

Use 34 degree, paper collated, clipped head nails. They were always the most common strip nails years ago as Senco, Paslode, and a number of other nailers used them.

If you mean the plastic top for the gas cylinder you should get 1 with every box of nails containing cylinders purchased.

if thats a hitachi nr90ad, then you should be using 30 degree paper taped clipped headed nails,, never use plastic or wire collated nails in that gun,, some boxes might say 30 up to 34 degree and thats fine, also in a lot of areas clipped headed nails are no longer code, so they made the nails full headed, the head is just off set,,

Yes, BUT the paslode Round drive nails and known to cause driver jamms due to the excessive paper used in the spacing of the nails.

www.powertoolspares.com sell parts for hitachi nailers.

The link below will take you to (toolpartsdirect). This is where I purchase all my parts. They are quite handy because they sell most all popular brands/parts. Good Luck with your repair.
http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/hitachi/NR90GC

Check and make sure your using the right nails. Are they held together with plastic? This should be a 21* full round head nail with plastic collation