Ah the dreaded TRANS PROGRAM.
Had this 2 years ago when I bought my '89 535iA.
Traced all the possible electrical faults and eventually (after being faced wiht a $3500 trans rebuild) traced to reaplacing the speed or pulse sensor in the trans.
Total cost AUD$200.
Hwn you get TRAN PROGRAM, the trans goes in to "Limp mode" which isessentially 3rd gear.
My symptons only occurred about 20 mins after the trans warmed up.
Check the link as suggested to Bruno's site.
Good luck.
Cheers,
Trevor (Sydney)
E46
00' 323iA (70,000 kms)