Transmission not shifting into 4th

Hi,

I have a 95 Grand Prix SE that will intermittently stay in third above 45-50mph. I had the transmission fluid flushed and filter replaced in June, and have put maybe 1000 miles on it since.

While doing 65mph last night on my way home, I noticed the tach stays at about 3k to maintain speed. When the cruise control is off or when I'm not giving it gas, it drops down to 1k. The car drove like this for half of the trip, and after I stopped for about an hour and a half, it drove normally the rest of the way home.

This has been happening occasionally for several months, usually on warmer days or after the car has been driven a while (I had the fluid changed, but it didn't help).

I've been looking around here for similar problems, but I haven't found anything. It's not slipping or stalling, like most of the other problems I've come across.

Does anyone have any ideas? Thanks in advance. :)

I believe your 3-4 shift solenoid is going bad