93 4.3L Revs jump when hot and on highway

Hi all:

First off, I want to thank everyone for the great advise given on this forum. I've used it for many different vehicle makes and have always had good luck finding answers. Now it's my turn to ask for help.

Just recently I installed a larger radiator (old one was cracked and got lucky and found the larger one) and a 180* thermostat on the blazer. After a couple of days of driving in the 110+ degree weather here in Phoenix with the AC on, I noticed the RPMs would jump up immediately after driving 70+ then decelerating. The RPMs would stay high for about 10-15 seconds then it would return to normal. This vehicle doesn't have a tach so I can't say how high the RPMs go but it is easily over 1000rpm jump.

I bought a 195* thermostat and installed it but that didn't make a difference.
I also added more freon since it did need it but again it didn't help.

Any other ideas? I really appreciate any suggestions you all may have.

This is a 93 S10 Blazer Tahoe 4.3L CPI, 5 speed Manual tranny 4x4.