Tempo changes - rallentando

Mike, you've been here for awhile and have asked a few questions, and we almost always have to ask you which program you're using, what OS, sound card/audio interface you have in order to better answer your questions.
If you would go to your forum profile and add that info to your signature, we won't have to ask everytime since it will be written at the bottom of your posts already.
Thanks.
 
If you have a good idea where these tempo changes are going to occur in the song, you can change the tempo before recording by going to Insert>Tempo Change. Just change the tempo at the measures you want them, and SONAR will change it at those places.
If you have a song that graually slows down, you change the tempo to a lower number a measure at a time for several measures.

Tremor
 The problem is my difficulty in playing the part of the tracks which I know should be "slowing down" with sufficient correctness whilst still at the higher tempo. Also if I record one track and apply the gradual change, it's very difficult when recording the next track to "sync" it to the existing one which already has the tempo changes applied. 

I'm pretty sure I would have a problem with that as well.
Trying to play something different from what your brain wants to hear would be very challenging.
On your second note, that would be nigh on impossible, I think.
As you attempt to sync the second track, the first would change proportionally.
Expanding on Greg's approach, I would probably lay down a click track using SI Drums, or SD2.
Then open the Tempo map, and adjust accordingly. You can even draw in the tempo changes if you wish.
Based on your equipment list (thanks for that), I am guessing you are recording mostly MIDI tracks?
Make a few dry runs, and adjust the tempo until it  fits what is in your head.
Once you have that baseline tempo mapped out, you can follow it, and play the way you want to.

Hi Guys...and thanks for the ideas. Kalle, you've had the best idea so far. Giving the tempo more clicks in the track definately makes it easier to follow. I've used a drum track with a regular 16 part beats and quantised the measures to tighten them up. Seems to be easier to follow. Yep it's mostly midi tracks - I don't sing so good any more and arthritis put paid to any serious guitar playing - part of the "middle-aged amateur" syndrome!!