Thanks Dave,
I did some extensive searching after posting my reply and found some info that was helpful in repairing my issue (so far anyway, I've done about 20km since fixing and no issues yet). I thought I would return and post some info on how I fixed (including a link to the page that was most helopful).
It appears that this issue affects the US model Frontier of a similar age as the early D22's. I found the below link which refers to replacing the the Transistor chips in the speedo unit (code is NTE152 Transistor). Reading through other links, it appears that some have had success in redoingthe solder on said transistors, as well as redoing the solder on the 3 IC's located in the speedo unit.
The word is that these were not done correctly at manufacture and that over time the solder becomes brittle and stops doing its job (i'm not a real techy bloke, this is the best i can describe it!)
http://www.4x4parts.com/ubbthreads/showthreaded.php?Cat=0&Number=602001&page=0
I performed this today and reinstalled and looks good so far!!
Some tips:
1. To remove the speedo unit, you will have 4 screws on the back of the unit to remove first.
3. In order to pull the guage from the cluster you have to remove the 'face' of the guage. To do this, you first have to remove the needle. DO NOT do what I did and just try to pull the needle off its pole (I'm not real tech savvy, so don't what the proper term for what this is). Attached to this pole is a very fine copper spring that will get all out of wack if you just pull on the needle (My speedo now reads about 20 under real km/hr since I stuffed it up - not too big an issue, as all other guages and odo work perfectly and I use a GPS anyway).
Best way would be to get a very fine set of needle nose pliers under the needle to grab this pole and 'twist' the needle off.
4. The link above shows the location of the transistors quite well. Refresh the 6 sections of solder that hold them in place (I also added some solder to refresh the joins that you'll see as well).
5. Redo the solder on the 3 IC chips as well (there are quite a few of these to do!). Once you have the unit out they are pretty evident where they are by looking at the front for the codes (IC1, IC2, IC3).
6. Reassemble and install them and hope for the best!!
If this doesn't work, You will likely have to replace the transistors by following the guide attached above.
Anyway, long post but I thought I would put up some info here for others to find suffering similar issues!!
thanks