DVD upscaling; motion movement pic quality

Can somebody explain to me why that is: When I upconvert my regular DVDs to 720p, the motion movement pic quality is extremely poor. What I mean is when there is a scene where the camera is moving from left to right, ie a landscape, the motion is not smooth at all; the movement starts, stops, moves little bit, stops, starts, stops and so on. The movement is not smooth at all. All other scenes are quite ok until there is a movement across the screen. It gets worse when the movement is quicker.

I had this problem when I hooked up one of the earlier Oppos to my Panasonic AE700 pj couple of years ago and now I'm experiencing the same with my Samsung DVD-R136 (not the best piece of equipment I guess) with a 46", 2y old Samsung 720p LCD.

I always upscale thru HDMI hooked up directly to the LCD or pj. Audio runs thru my Rotel RSP1098 processor.

Is this a common problem? Are there players, chips, processors I should look for? What should I look for in my next 1080p upscaling DVD player. I got quite a DVD collection.... no bluray discs yet.. thx