I think u should gv test again n again & see ur scores whether they are showing this much difference @ home too.
give all KAPLAN
give all GMATprep.
These 6 test will show u ur performance & u can judge by them why u r getting so much variations.
1) prepare your chart
2) see ur verbal & quant score seperately
3) in which section u r getting more variation in scores.
4) if in both then u need to focus on questions
5) if 1 of them then u need to buck up on that part
6) if u r getting good scores @ home then u may be getting nervous @ test center.
7) last but not the least have a microscopic view ( if u r going wrong, u r goin wrong in which section, verbal,quant if verbal then sc, cr, rc if sc then parallelism, modifiers, pronouns, comparisons, idioms )
I was also feeling nervous when I was going for GMATprep ( not GMAT) for 1st time coz I knew dat I wont b doing good in verbal. Then I scored q47 v21. Then I rescheduled GMAT. On that day, I realised when we are obsessed with something consider it as a "DO OR DIE SITUATION" & hv fear that we won't score good, our confidence simply shatters. So, be positive n simply learn by posting questions n clarifying your doubts. Note them whenever you feel like. Have 1 diary with u. Don't let ur doubt unsolved. Ask them till u feel satisfied. Remember-"THERE'S ALWAYS ROOM FOR IMPROVEMENT".