Millee,

Since you've had a mechanic look at it already, most of the obvious problems
SHOULD be eliminated.

I'd be looking for: bad connections (pull each electrical connector and coat
with dielectric gel to preserve a good contact); weak or missing grounding
connectors (same as above, but will need to be unscrewed, cleaned, and
reattached (including ECU ground)); adjustment of AFM (should NOT need to be
readjusted for altitude); vacuum leaks (listen for any hissing noises).
These fixes are "free", since there are no parts replacement involved.

Disconnect coolant temp sensor and see if any improvement; if so, replace
the sesor (about $5).

Next is O2 sensor in the exhaust system.  If you can borrow one from David B
(lives in RI) or someone else, just attach it electrically without screwing
it to the exhaust and see if any improvement.  If so, replace O2 sensor with
new - say thank you to David or other.

After these steps, you're beyond my knowledge.

Good luck,

Karl Wolz