It’s strange that it stops. Your Abis satellite is certainly not 2Mb/s, you’re just buying a certain share, equal to 640kb/s or so.
And 2TS in Edge on the radio interface are using up to 9 Abis ts (9 * 64kb/s). If you add a 3rd one, it’s probable the Abis link becomes congested, therefore reducing throughput on PDCH.
But why does it stop, i don’t know. Probably vendor-dependent.