I had an almost identical 'napping' episode with Dolly when I had only own her a few weeks. Her chosen method of evasion though was a swift reverse backwards like the the devil himself had sprung up in front of her!
Tried all the usual 'correct' stuff - even sitting her out but was on the brink of phoning home to get the 'heavys' to come down an chase her along. Just tried one last riding school kick and a swift whop across her rump with my crop and forward she charged. Must confess I was somewhat nervous for a moment or two as to how she would react to this firm stuff from me!
She did attempt to repeat this at the same spot (couldn't tell you what if anything was her problem?) - but was ready for her and riding school kicked almost before she had moved a muscle in reverse!
Have to say she hasnt done it since - I am convinced it was a complete test of her new owner - think she wanted to find out if I was really as soft and useless as I had led her to believe!
Be ready for her next time at that spot - and shove her on before she even knows she's going to nap herself.