skips in third gear

i have searched alot for this and found similar problems but not the one im having.

I have a 92 4.0 157.000 miles. When I get up to about 55-60 mph or when passing on highway, engine skips alot. If i let off the gas and it shifts into fourth gear it runs fine, but it never stays in fourth gear for long so this happens all the time. I changed the cap, rotor, plugs and wires a week ago and i did not help at all (idles smoother though). checked codes tonight and all i got was battery disc. in last 50 starts and fault in fan relay circuit. Any ideas on what could be wrong?

I would suspect the TPS(throttle position sensor).

Replaced the tps today and it still does it. Any other ideas?