I have a 98 vw passat and sometimes take iff slow why is that ??

SOURCE: VW Passsat - 98 -

First check the coolant overflow and add a little water if low (if you have antifreeze then a pint of water will not dilute it enough to matter).   
Go to a parts store that will plug in and read the OBD II codes for free (autozone among others).  My dealer change $50 just to read the codes.  BTW you can buy a simple VW code reader for $50!   ABS system will give specific codes as to what is wrong.  Coolant warning light is usually either low coolant in resevoir or overheating.  If the coolant light comes on when you first start up,  it's probably just low coolant level.  My wifes 98 Passat and my 2005 Jetta TDI both did this for a slightly low coolant with no detectable leak.   

If you have to add coolant more than once, you may have a small internal leak (read in the cylinder head) which you will not be able to find.   A Mechanic can do a pressure leak down test to find out.  This is one of the 2 times in 35 years where I used Barr's Stop Leak on our 98 Passat with 120K miles for this slow internal leak symptom ... it's 20K miles later and I have not had to add coolant since the Barrs.  I generally hate these fix it in a can type products, since many of them are snake oil,  but Barrs can work without side effect in some cases.  I have it from a reliable source that the manufacturers use a smiliar substance on new engines now that most cylinder heads are aluminum, but I have no way to verify.
good luck ..ajm