Corsa-B.uk - Vauxhall, Opel and Holden Corsa B Owners Club

Hi guys, help please, my hair is getting thinner & greyer by the week.

An ELM327 reckons it's a X14SZ and gives a speed sensor error, paperclip gives 0500. New sensor fitted, same.

From cold the car works perfectly. When partly warm and going downhill I usually get the engine light for maybe 1 to 5 mins.
If I then do 10-15 miles the speedo dies to zero and the MFD (press both buttons) also shows zero. From here on, when stopping at lights the idle will plummet to ~500 but often recovers to 750, sometimes stays at 500, other times falls to 750 as normal, could be either. Switch off for a few mins, back to normal but all faults return fairly quickly.
Its only done 65k, I'd hate to dump it without a decent attempt at repair but its worth so little I may have to.
Any suggestions however crazy gratefully received.
TIA.

same engine as me im sure;) had a few beers so may not be accurate but I had rough running slight detonation on light acceleration and cutting out especially after engine was warm and slowing to a stop it would kangaroo and stall try paperclip test but if its what my prob was it may be egr valve tried clean it etc and would not solve prob so tried un plugging it and hey presto car ran perfect lol apart from eml constantly on. that was in august last year a.d still runs super nice apart from eml on all time