Controlling Pace and Keeping on Topic

Some of my biggest customers are naturally some of the most difficult to deal with. Most times the difficulties are relatively minor and can be handled with a little "baby sitting" & good customer service. With a few of my customers I really have what I feel is a difficult time (they might not feel the same because I do quite well with them). The problem is basically this. Their personallities are fast paced often high strung and somewhat like a child who's had way too much candy before bedtime. Before I can even sit down in the office to talk with them I've been asked 5-6 rapid-fire questions, none of which had a pause after it to allow for an answer. I find quite often that this can throw me off my game and I leave feeling like I just made a terrible sales call. I normally handle this situation by letting them continue on until I can get a word in and address 1 or 2 of the questions mainly trying to slow the pace of the sales call while still letting him know I was listening. Then I'll usually try to switch topics to why I thought I was calling on him in the first place sometimes only to be bombarded with another round of questions. Is this just a control issue? Any suggestions on how to better handle this type of customer while still keeping the call on topic?