Nick
Don't know if I am restating 'ancient' history here but, before trying faster memory cards in your 990 have you checked the firmware version yet? You do this by holding down the Menu button as you switch your 990 on. If it is version 1.0 then this will slow your camera down a lot. If so, you need to load in, via your memory card, Firmware Updater Version 1.1 which is available as a free download, with instructions, from Nikon:
http://support.nikontech.com/cgi-bin/nikonusa.cfg/php/enduser/std_alp.php?p_prods=1%2C8&p_pv=2.8&p_cats=186&p_cv=1.186
This almost seems to have a miraculous effect on the 990, as the write-up time to your card is much quicker, the focus much snappier, and several other things are much improved, and you can use the much maligned(I find it perfect if you just follow the correct procedure each time you connect it) Nikon cable release.
As regards memory cards, I find it best not to skimp on cheapos. I use Lexar Platinum and Sandisk Ultra. The 990 has an optimum in-built speed, and using modern day 133x speed, or whatever, won't make any difference as these fast cards are mainly aimed at the digital SLR market.
Also, I might be imagining it but, the fewer settings I have on Auto, it seems a tad quicker, perhaps with the camera not having to make so many decisions before actual image capture. It is also best, of course, to half depress the shutter button for each picture. I never use Programme setting, always Aperture Priority, with the aperture open to its widest to use maximum shutter speed....etc.
.....but I expect you knew all of this, anyway, and apologies if you did.
Steve