R33 speedo problems....advice needed

[stevo]

have had problems before with the speedo giving up now and again, but in the past few weeks it seems to be happening more and more often. it used to happen only after long journeys at high speeds but now at quick trip up the carriagewayeway and it goes bananas, 0-100 and back in an instant. ive done a little searching and heard about the converter behind the dash, is this likely to be what the problem is? i dont fancy ripping out the dash without being sure. if so are these cheep and easy to change?

and completly off topic, am looking to get car undersealed. should i let local cowboy spanner monkey at it, do it myself, or get in the pros?