Buster,
first off keep your head down and you'll pass the ATPL's. I had difficultys with some subjects but 1 year later I'm flying a B737 for a European Low cost carrier. Its hard work but you'll get there.
Two things to remember are
" Instrument rated examineers are truly picked "
IAS
Pressure
Instrument
RAS
Compressibillity
EAS
Density
TAS