I would suggest a vertical adapter and either an expansion chamber or a Palmer Stabilizer. The Palmer will even out your shot to shot consitancy quite well. As for barrels, the flatline increases range but accuracy is about the same. I would suggest the Smart Parts Feak system. It enables sizing the barrel to ball size increasing accuracy and gas economy. Indoor play usually yields better results than outdoor play. Do you run tank on your marker or in a harness? How many balls do you shoot per game? What loader do you run on your 98?
Here is a few links to cost effective products for your marker:
Smart Parts Freak Barrel Systemhttp://www.rockstartactical.com/index.asp?PageAction=VIEWCATS&Category=550Smart Part Progressive Barrel(well worth the price)http://www.rockstartactical.com/index.asp?PageAction=VIEWPROD&ProdID=2452BT APEX (similar to flatline)http://www.rockstartactical.com/index.asp?PageAction=VIEWPROD&ProdID=3034Remember that staying out of the oppositions range but being able to hit them is a tactical advantage.
Tippmann Expansion Chamber Kithttp://www.rockstartactical.com/index.asp?PageAction=VIEWPROD&ProdID=1757Keeps CO2 a bit more stable for shot to shot consistancy
Empire Reloader IIhttp://www.rockstartactical.com/index.asp?PageAction=VIEWPROD&ProdID=2291I use the Reloader II on my small electros (SP1 and Vibe) as it is cost effective, reliable, and easy to replace if broken. It is sound activated so every time you shoot it spins a few degrees to reload the feed neck and it doesn't run all the time like the annoying simple electric loaders.
We run at least two 98s in the group that rearely have problems. I would suggest a barrel like the SP Progressive first, a decent loader like the Reloader, and then the Expansion Chamber Kit. Aside from that switching to HPA/Nitro is the best thing to do for accuracy and consistancy. Seems some of us are stuck ith CO2 due to air availability. But HPA/Nitro is so much more consistant and cleaner for your marker.