4000 rpms

So, I started having transmission problems, I
changed the filter and restocked the fluid. 90% of
my problems worked themselves out after that. But
now, 4 days later, my RPMs shoot up to 4 on the
dash at 55-65. I havent tried running it on the hwy, i
dont trust it yet. But i'm just looking for some sort of
solution as to why my rpms sit at 4000 in town and
why it seems my engine is over working itself. (i
have a 95 Chevy k1500 4x4)
The truck has 212xxx miles on it but engine has
been replaced before. I don't know how many miles
are on the tranny.  I assumed it might be the
Throttle Position Sensor, But I havent changed it
because I don't want to change a part that doesnt
need to be changed. It shifts but my Rpms stay at
4000+ at 65 mph and over. Thanks