I'm afraid that if the official Skype app does not work then you are probably out of luck. Skype do not let third party applications use their service, so you are unlikely to find anything that works.
I suspect the issue is down to the CPU speed of the HTC Wildfire, which is only 528mhz. It's likely that the processor simply can't keep up with the demands of the application, which is why you get the stuttering, and why performance is the same on other Wildfire's.
Unfortunately you may have to stick with your other VOIP application.
Edit: There are a lot of posts on the Skype support forum where people are complaining of poor performance on the HTC wildfire. The Skype rep does say they are aware of it and trying to improve performance, so you may just have to wait for an update to the application sometime in the future.