You are not streaming, as transcoding is required to playback the AVI with the srt enabled. Wifi is typically a hinderance and you may want to see if you can leave the program paused for a while then fast forward to let the buffer build up on it. Don't use the go to option often if ever, so can't really be helpful there. Typically though hard wired, i can transcode AVI with SRT and fast forward at 30x just fine. If you can test hard wiring give that a shot as wifi is often a limitation.
Core2Duo E8400 (@ 3.0), 8gigs of Mushkin, Vista Home Premium SP2 64bit, 2.5 TB of Space, Wired to Linksys WRT54G wired to 80GB MGS4 PS3 on Sharp 32" 720p LCD TV