Your problem is one we all day everyday at the academies. In fact, your ball flight is typical of at least 60% of all social and improver golfers. So, although that doesn't solve your problem per se, it may give you some assurance that your problem is far from unique.

The first thing we have to do is close the club face at impact. Good teaching pro's always solve club face angle before swing path. Your "right" ball flight is cuased by the club face being open at impact. Now: 2 things can cause this. Either your club face is open at address (which is a common problem) or it may be square-ish at address and then fans open at impact.

Believe this or not, a weak grip actually has little to no effect on the club face angle. If anyone says that to you first, ignore it. They have got an up-to-date study of golf swing theories. That's an "old school" theory that has been disproved in more studies than there are days in the week. Provided that the club face is square at address at the hands rotate correctly through the swing, the club face will be back to square at impact.

To cure your ball flight is actually very simple:
1> Make sure your club face is square at address. Depending on whether you practice on mats or grass, there are various ways of doing this. Let me know which you normally practice off and I'll give you some ideas.
2> Do a drill to help you square up at impact. What you do is hit half swing using a 7-iron (or maybe a 6-iron) and swing to just past your hips then back into impact and through to a balanced finish. Do this over-and-over making certain that you rotate your hands and the club face square or closed.

Hope this helps you. Post again if you need more help.

__________________
Golf is easy ... once you know how.
Graham Arnott, teaching professional
Kelrosa Golf Studios
www.kelrosagolf.com
Class 'A' PGA Member
Full Member: World Golf Teachers Federation (GB&I)