Audi A4 temperature gauge 1998 model (S Reg)

Hi can anyone help out here, I have the audi A4 1.9 TDI SE model 1998 and recently the temperature gauge inside the car doesnt go above 70. When cold it starts at 60 then when warm stays at 90. The water levels are fine and the heat inside the car is warm or cold depending on what temp you put the climate control to. The air cond works fine also. And the fan behind the radiator is spinning constantly, which is normal I think. I am wondering if it is still ok to use the car as it doesn't seem to over heat when driving. If I stop at lights it does tend to go down slightly from 70 to maybe 65 ish, and then on driving again goes back to 70. I am just a little concerned about this and would like advice before walking into a garage and they give me the twadde of it needing this and needing that when it doesn't. Any advice would be much appriciated. Thank you.

11/10/2006. Had the Temperature sensor gauge renewed. Cost £70.00. The old sensor unit had failed which was causing the problem of showing no temperature on the dash board.
Thanks for all the feedback on what you might think it is ( NOT!) 99 views and not one person advising anything.