I'll give some background: Today I changed the oil (Penzoil 5w30, 5 quarts + 3/4 of a can of Engine Restore for 4cyl cars that was laying around. It's a little high, going to drain some out tomorrow.), oil filter(AC Delco pf454), and thermostat (Hypertech 160'). I'll be ordering a 1LE intake elbow because I saw the end that connects to the TB is torn and ripped on the bottom. Four days ago I used 5/8 - 2/3 of a can of Seafoam through the brake booster vacuum line, and the the rest into the crankcase. One can into the gas, too. Let it sit 15 minutes, ran the hell out of it afterwards. I ended up not removing the o2's before Seafoaming. Zero problems with the car until I changed the oil and thermostat today.

I changed the thermostat while the oil was draining, which means the car was sitting at an angle... and I lost at max a quart of coolant from it. But it dripped down all over the place so I was worried about the opti because I don't know exactly where it is, heh. Anyway, the car started up fine so I doubt that's part of my issue.

The Problem: At a 50-55mph cruise (about 1400rpm - right where I have been able to feel a very slight vibration through the accelerator from the stock TC and 11 year old tranny/motor mounts I'm guessing) in my 3.23 4L60, the car "hesitates," I guess, to the point where I feel the car is starving for gas. Pedal feel is like it's sinking/disconnected - I'd imagine it's like having a very high stall TC. I hear/feel a little bit of a stumble/stutter from the engine. The tach will not climb, nor will the speed increase, until the car downshifts into third. The problem also tends to appear at lower speeds at the same rpm while cruising(1400-1600 when I start to give it more gas), but much less in degree and harder to reproduce. The rain here isn't helping either. No SES light.

Tranny fluid isn't burnt. Previous owner (above average maintance care for the car) dropped the pan in early March to have fluid and filter replaced. Oil pressure after the oil change has bothered me though, because it has been about 15psi higher than what it was before (10w30 was in it) throughout. It was a minimum of 60psi for hot cruise, cold start, hot idle... but now it's almost normalized back to how it was before, 20-25psi at hot idle; 40psi cruise; 60psi leaning into her a lot. Doubt this has any bearing on it, however.

From what I can tell there is no upper-end loss or misfire (roads are wet). The car runs fine from idle to whatever RPM I level-off at, regardless of how I get there. Problem shows up when I've been at the rpm (1400-1600) then start to try to speed up to feather from a 45mph to 60mph type of change. Slowly revving in neutral seems normal (never done it before), though I do get a little bit of virbration at 2k rpm that quickly comes and goes, and I have to be delicate with to find.

I searched through over 20 pages of search results. Closest problems I found were:
http://web.camaross.com/forums/showt...light=1500+rpm
http://web.camaross.com/forums/showt...light=1500+rpm - I believe Fastbird has a different problem, but Zybermunster has the same issue as myself.