Most bizarre thing

IN the past 4 months, my 1999 sebring convertible(JXI) has done a weird thing twice. Ill turn the car on, and for like 10 seconds or so, the engien will get louder than normal and speedometer will go like haywire(it will say im going like 60 mph when its still in park). Also, 1 of those 2 times, i tried putting the top down while it was doing it and it wouldnt go for like a minute. I would turn it off/on again and it would be fine(both times) like nothing had happened. None of my friends have any idea what this could be. Any ideas on if its serious?