faulty transmission temp. reading

[Bob & Carol Howald`]

Last year while on a trip my transmission temp suddenly went to 300
degrees. I immediatley stopped and checked it out and determined that
the temp was in the normal range around 170. The gage stayed at 300
degrees the rest of the trip. I ordered a new gage from BB and
installed it. After that the temp would go to 250 degrees and stay in
that area. Again I assured myself that the temp was in fact normal
170.Thinking that it could be a bad sending unit in the transmission, I
changed it this week when I had the pan off. I started the coach up and
the transmission temp. went to 230-260 in about 5 minutes. again I
assured myself that temp was in fact normal about 95 degrees just
idling. I rigged up my old gage under the coach at the sending unit
using different wire from sending unit, new ground wire and a hot wire
from the battery. It seemed to give a normal reading at this point.
What do you suspect the problem to be? bad ground at the dash? bad wire
from the sending unit? Any suggestions would be welcome. Thanks Bob
93pt Illinois