Georg,

I purchased my Firebird ('91 Formula) in 1988 with this problem. It had
been initially, and incorrectly diagnosed as being a fault of the
throttle position sensor (TPS). Upon further diagnosis, the fault was
actually with the injectors themselves. It seems that the injectors were
intermittently sticking; most noticeable during throttle transitions and
at small throttle openings.

You did not say which engine your Firebird has, but if it is a TPI V8
(5.0L or 5.7L) it will have 8 separate injectors (1 for each cylinder).
I am not sure of which fuel injection system is used on the 6 cylinder
models. Either way, a technician with an engine analysis tool should be
able to diagnose if the injectors are the problem quite quickly. The
injectors provided on the TPI engine Firebirds and Camaros are cleanable
at a shop with the injector cleaning equipment, but it is not
recommended to abrasive blast clean them as the plating may (read will)
be removed during the process. After which time, the injectors
themselves will rust internally, making your problem even worse. If
cleaning the injectors does not solve your problem, you will need to
replace them - be warned the stock GM injectors are very expensive. Also
note that most shops charge for about 6 hours labour to disassemble, and
reassemble the TPI plenum and fuel rails.

Franki

   Pull the fuse  for the 'ECM'  for 1 minute.  Which should  clear any
trouble  codes, and programming that the car can 'learn'.  Then drive it,
and  check  for  trouble codes.

   If no codes  show  up, you should get the code for 'ECM'  ok. If you  do
not  get this  code, you have a ECM problem.  Which may be as simple as a
broken wire  or bad contact.

   From this side of the pond it almost sounds as if the cruise  control is
trying to activate  when you are braking, then is leaking vacuum  when you
are accelerating.
Charles