One thing I'd suggest is setting Xms to 4096 as well.
Also, apparently this is the error when saving:
1174717 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.82031035
1174733 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.83278656
1174750 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.84370327
1174767 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.8592985
1174783 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.8733342
1174800 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.88737
1174817 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.90452474
1174833 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.92012
1174850 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.93415576
1174919 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.94663197
1174930 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.9575486
1174940 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.9669058
1174950 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.9762629
1174967 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.9840605
1174983 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 0.9980963
1175000 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Saving progress: 1.0
1175000 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Releasing context
1175000 [Thread-13] INFO com.fs.starfarer.campaign.save.CampaignGameManager - Released context
1265074 [Thread-4] ERROR com.fs.starfarer.StarfarerLauncher - java.lang.RuntimeException: No OpenGL context found in the current thread.
java.lang.RuntimeException: No OpenGL context found in the current thread.
at org.lwjgl.opengl.GLContext.getCapabilities(GLContext.java:124)
at org.lwjgl.opengl.GL11.glDeleteLists(GL11.java:714)
at com.fs.graphics.util.GLListManager.invalidateList(Unknown Source)
at com.fs.graphics.util.GLListManager.nextFrame(Unknown Source)
at com.fs.starfarer.combat.CombatMain.main(Unknown Source)
at com.fs.starfarer.StarfarerLauncher$1.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)