Aspiration / Asthma Food-rewlated?

I had a fairly serious head injury over 2.5 years ago.  One of my symptoms was a slight difficulty swallowing.  At the time, with the VF swallow study, they noticed that I had some difficulty with the food passing through the sphincter (?).   This has since resolved.  However, coincindentally or as a result of this, I have developed what I think is aspiration problem.  In addition, I have been diagnosed with asthma.  

This is what happens, and it is beginning to happen more frequently.  When I drink soda, particularly colas, or most times when I drink milk products, within 5 minutes, I get a productive cough.  Sometimes I can actually bring it up, and its is usually clear, but most times, I can feel it clear my lungs and it never comes up.  This can last for 20-30 minutes, and then it goes away.  I have tried to avoid these foods, but it also happens intermittently with other foods.

I've had a VF swallow study recently, and it was negative.  They suggested having a endoscopic study done, and I'm starting to think that I need to do this.  My PC doc also suspects I've got sleep apnea (I'm not obese, or a smoker).  I declined the sleep study, as I've got plenty of other health problems to contend with.

Does anyone have any suggestions for me?  My mom recently died of COPD, but she was a long-time smoker.  I was raised in a household with two smoking parents, but have not been around second hand smoke regularly for almost 30 years.  No family history of asthma.

I hope I'm in the right forum

Thanks in advance for any help anyone can give.  I did do a search here, and came across a similar situation, but not exactly.