[V6-12v] Q: 3700 - 3800 RPM Check-engine light

[The CyberPoet]

I haven't popped by my mechanic's to pull the computer codes yet (he's 
a ways away), but swapped some vacuum hoses last week and now I'm 
getting a check engine light specifically somewhere between 3700 and 
3800 RPM, irrelevant of gear or other circumstances (engine cold, 
engine hot, after a mile or after 20 miles of driving, etc). Anyone 
have a clue?

Cheers
=-= Marc Glasgow