Hello Michael -
Thank you for your reply / info...
The sensor is acting oddly for a start Michael - given that I would check that out first before anything else.. Check the wiring to it- they are known to get damaged / burned - so check that. Also - check the plug connector where it connects to the loom - often the pins get corroded - causing signal failure... Check that too.
If that all checks out as ok, I would suggest to replace the sensor Michael and I think you will find that cures this.
Just also check for any inlet air leaks - any un metered air can affect the running at higher revs - check the pipework etc around the engine / breathers etc.
So long as the engine isn't actually misfiring Michael, thats what I think this will be.. certainly it needs attention anyway in that regard before anything else.. If you do have to replace the sensor - don't use a cheap universal one - they give more trouble than they are worth - you can get a good sensor at a good price HERE
... Genuine Vauxhall parts - cheaper than your local dealers.
So thats the way forward on this for sure now Michael.
Im very pleased to have helped / advised you - If you need anything else at all - by all means get back to me - Im always happy to help more if needed..>> Meantime >>>
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Kind Rgds, MIKE.