Help!!!

Just bought a 91 galant gti from a friend. Was told that the cylinder head was replaced with that of a Turbo 4g63- valves, cams,etc.. as in the complete cylinder head. Are there any benefits or downsides to this modification?

One time, after doing some "redline" shifting, the check engine indicator just suddenly flashed. Napansin ko umiilaw yung indicator kapag pa-minor na yung auto around 2,500 rpm. Mawawala ulit kapag pumalo ng 3,000 rpm pataas. On and off yung indicator. Inconsistent- minsan sumisindi, minsan naman hindi. Inspite of it, wala naman akong ma-feel na nagbabago sa takbo nya. Smooth pa rin yung idle, ok pa rin ang pick-up nya across the rpm range.

Anyone encountered this problem? What could be the problem?
Thanks.