Engine management light on when doing 70mph or wanting a bit extra power


I finially took my sporty efi, for its first drive today after regrinding the valves, and fitting a new cylinder head, around town it was ok, but on the motorway, i managed to get to 70 and the engine management light popped on when i came off the accelerator, it went out, i also had the fortune of following another sporty, which left me standing, and accelerated away up a hill.
I put it down to a couple of things,
1 a blocked exhaust
2 timing still out
anyone came across this problem?
or any other ideas?