DB MSSQL Transactions

I am currently experiencing a transaction problem with the PEAR MSSQL driver. I switched off AutoCommit to enable manual commit/rollbacks. I am running (at least) two transactions concurrently, with each transaction being multiple Update and Insert statements. At the end all insert statements are committed, but the Update statement is sometimes ignored (not committed). The update statement simply increments a counter. That works fine most of the time. I was able to extract the relevant pieces of the code to simulate the problem. When I replace the PEAR functions/class with the standard PHP MSSQL statements, everything works fine. A weird but reproducible problem!