1.9L Turbo off - eninge misfire's at higher RPMS

[Golf/Jetta]

1.9L TD, 97 Jetta
complete head rebuilt last year
no leaks nor oil missing/coolent
new injector's this year
glow plugs changed two years ago.
timing on the button
compression 390 - 410 PSI
no blue, white, black smoke

running with the turbo its fine @ higher RPMS but when the turbo's waste gate is opened (off) the engine misses at high RPMS..

i had the air box off during the winter months (conned filter) didn't misfire with this
then put it back on during the summer (K&N filter) and it misfires

what gives?
thought?