That is a tough one to tackle. My only idea right now is to dig to the pipe, cut it, and install some cleanouts. Once you have access, you can use any number of methods to inspect or clean it out.
An off chance is the bubbles represent a high area in the pipe with trapped air, lessening the flow. Get the air out, and it may help? Pull a strong siphon by attaching more pipe on the end and downhill somewhere, and it may flush whatever is blocking you.
