Two consecutive quarter notes in a track are not playing back smoothly

I can't play keyboard instruments well so I use the staff view, rather than my midi controller (A300PRO), when writing new tracks.
 
On one of my recent efforts I have a problem where 2 consecutive quarter notes do not play smoothly but instead produce a slight jerky movement. It may or may not be relevant but the 2 notes are F5 and G5.  It is an AABA sandwich composition and so this occured 3 times over the whole piece. I keep deleting the relevant measures and redoing them but the 2 notes still do not run smoothly. All the notes in the rest of the composition run smoothly.
 
Has anyone got any ideas please?
Alan 

What instrument is it? If it's a synth it could have something to do with the legato parameters if it has those options.

Thanks for comments to date, I appreciate but still need further help
 
I'll try to clarify further what I have done as I think I had tried all your suggestions except off setting the second note.
 
Quantisize may be necessary when recording from a MIDI instrument but I am entering notes using the draw tool and clicking on the stave at the required position. If you do not hit the correct point on the stave you can tell immediately as you have a visual indication on the staff view. Hence I think there is no need to quantisize as there are no timing errors. (Codamedia)
 
I am using the Cakewalk TTS-1 synth.
Have already tried shortening the length of the 2 notes. (Bristol Jonesy)  I have tried deleting the notes from the stave and replacing them. I have tried deleting the measure then reinserting it and entering the notes. I have now tried offsetting the second note (Codamedia). None of these actions have been successful.
 
I'm struggling to understand legato parameters. Based on my further information do you still think they are relevant? (Sharke)  
 
There are 3 notes in the measure causing the problem. The first is a half note (or minim) and the second and third are quarter notes (crotchets). The jerk is as the second half note follows the first half note.
 
A strange thing is that I have at the end of the melody track I repeated the errant notes in 3 measures in the tracks and they play back perfectly. So I tried copying the measure that worked and pasting it in the measure that was causing the problem. On playback it was still jerky.
 
  
Any further suggestions please?
 
Alan 

Regarding the legato settings, it was a long shot, but worth asking. Since you're using the TTS-1 synth it's not really relevant. As an example, I use Monark a lot, it's a monophonic Moog emulator. There are legato options for envelope retriggering (Never, Note On, On/Off) and depending on how I have that set, sometimes a series of notes will sound jerky because of the way the MIDI notes of the performance overlap.