emission control warning light

[lrose]

Hi all,

New to this forum so please be gentle :-)

Recently had a top end rebuild on my 2000 yr Boxster S (2 valve springs had broken) and car running fine again until last week when I started it up and noticed it felt a bit "lumpy". It was running at quite high revs which isn't unusual when cold, so I didn't think much of it. 5 mins down the road and the emission control warning light came on (only found out it was this when I looked in the manual!). I drove back home as didn't want to damage anything - recent problem still firm in memory! I called the garage who had done the work and they said it may be a faulty oxygen sensor but that I'd need to hook it up to an ECU to confirm. I called a Porsche specialist near me and they said to drive it down as wouldn't do any damage and they'd have a look for me. The diagnostic said there was a fault with cylinder 4 so he changed the coils over on 4 & 6 and put a new spark plug on 4. Reset everything and appeared to be ok. 3 days later and the car suddenly felt lumpy again and the warning light came back on.

Anyone any ideas what it could be? Do you think it's an isolated problem or could it be related to the top-end rebuild?

Don't know whether to take the problem up with the garage that did the rebuild (200 miles away!) or take it back to the my local garage.

Any advice / thoughts welcome!
Thanks.