my van starts to rev up when am driving?

hi, i have a 2003 vaulhall vivaro 1,9.while driving the revs can just start to climb, take foot of the trottle and down they go. this all started after( i think) the altonator died. new one fitted, also some times the speedo can drop to zero miles for a while/ or just a fue seconds.other than that van sound and well maintained 110k used mostly for social use, no heavy work.( bikes for scrambles)