We have a Janome sew precise machine that we bought new about two

I would take it in for repair or adjustment.

take it in for a service specifically the needle/ hook timing
make sure that the thread tension is correct

Take a photo or the actual machine to a Janome Dealer and they should be able to point you in the right direction.

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This is a sewing machine we don't have in the US, so I'm guessing here, but I think the stitch pattern is unbalanced. On some machines, there's a dial to "balance the patterns", which you can turn to restore symmetry to something like a buttonhole. Often you need to automatically stitch out a pattern in order to reset the balance knob.
If your machine doesn't have such a knob, it's time to take it in for a lookover.

Is the stitch selector set to a straight stitch or is it possibily in another stitch pattern but with the width turned down to zero?

If machine is set on a straight stitch but is giving varying stitch length for no reason then it could be that the feed dogs are not functioning correctly, could be out of time with the needle or very worn. Or are you pulling the fabric through rather than letting it feed through. Also check the pressure being delivered by the foot pressure if the machine has a variable adjustment for this. It may have just been turned down for very heavy fabric feeding and needs to be turned up to normal setting again.

Feed dogs can wear - replacement parts may be available. Just remember to search under either Janome or New Home as I believe New Home was bought out by Janome some years ago. Certainly New Home was a common brand in UK in the 80's.

You might find parts on www.sewingpartsonline.com or http://www.sewingparts.co.uk/

I hope that this helps you out.

Hi
You can download some partial manuals from the Janome site.
This is for the Janome D'cor Excel Pro 5124:
http://content.janome.com/documents/File/ManualFile/DecorExcelPro5124.pdf
Which should be similar to the ohter model number you mention.
Hope this helps!

on my machine, as janome, there is a feed dog lever behind the bobbin are, and a pressure foot adjust on the top of the machine. But idf you have touched those perhaps something is catch in them and thats why it keeps happening ??

Just today  i got out my little janome i bought years ago and it is this model.  There was no "manual" with the machine.  All that came with it is a large 20X14 two sided print out.  I can scan and send this to you with an email address.  

Her is the manufactures website. Maybe you can find a manual from them?
http://janome.com/index.cfm

You can adjust the pressure foot(not footpedal)which puts the prsure on the foot by the needle. Also try altering the stitch length. If the stitches are skipping on silky fabric or knitted fabrics try using tissue and sewing onto that directly then tearing it off afterwards.