I agee with Edison, more likely your T-1 is congested. If so, the "no fair-queue" will have the interface running with FIFO which usually causes erratic performance when bulk and interactive traffic are mixed. (What the other side of the link is doing for queue management is also often important.)
Besides total traffic monitoring, you can look at the interface rate when the link seems congested and see the inbound/outbound rates. Further, suggest you set the load interval to 30 seconds.
Another indication of congestion is seeing many recorded drops on the interace.
As for the MTU, being too large, the only negative effect, I believe, is suboptimal buffer management. You could set it down to 1500 if you know your packets never exceed that. (You don't, however, want to be smaller than you packets can be or you'll create packet fragmentation.)
I also see you're doing frame-relay. You might also have a situation where your CIR rate is much lower than the line rate and you're getting frame-relay drops.
