Yeah, my speed changes every bump I hit because it causes my throttle hand to move.
Here is a link to a simple home made throttle lock: http://www.scribd.com/doc/8931483/Cheap-Cruise-Control-DIY
And here is a commercial model:http://www.amazon.com/gp/product/B000GZPGYE/ref=pd_lpo_k2_dp_sr_2?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B002EYZZSC&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1BR6C18H6W2C2YH04WJM
Basically the same gizmo.
Some guys like throttle rockers: http://throttlerocker.com/, but I like the lock because I can take my hand off the throttle when it gets tired.