Got "Could not send Message." Error from Marketo Soap Api

Dear Sir or Madam,

I am a software engineer from Sumo Logic Inc. Recently when we using marketo's SOAP api to send message to marketo service,

(exactly speaking, I use the "syncLead" api as developers.marketo.com/documentation/soap/synclead/), I will occasionally get some error messages like

- "An unexpected javax.xml.ws.soap.SOAPFaultException was thrown."

- "Could not send Message."

which is really bothering our development process.

Since this issue only happens randomly sometimes, I just wonder if this is possibly because we were somehow being throttled by marketo's api service?

Or, could you please share some guidance I can follow to find out more details so that we can figure out the root cause?

Thank you very much and appreciate!

Bruce