HELP !!Transmision probs on a 91 Galant 4AT

Help!!

I have a question regarding a mitsu 91 Galant GSR with 4G63 DOHC and 4AT.

I only purchased the car a few weeks ago and it's been fine till 3 days ago.
there was a death rattle from behind the timing cover. closer inspection
resulted in replacement of all belts and the three idler bearings.Once the
mechanic finished with the car the transmision, which has been opperating
fine for the past 5000Km, went into limp home mode. I'm told there is a way,
but no one knows how, to reset the tranny so it no longer does this.

Car particulars
it's an Australian delivered 1991 Galant GSR Hatch with 149000Km and a
service history. recent replacement items include an engine managment unit,
coil pack and plug leads. economy and power lights work on the dash as does
the O/D light. just wont kick down and takes off in 2nd or 3rd gear only
(not sure which)

any advice would be appreciated

James