maybe a long shot maybe the pully is freezing up, not acually freezing but you get the idea, maybe loose, or perhaps it's on it's last leg. have you checked for any leaks? keep us posted
Could be the rack/pinion. On my 90 years ago the steering felt funny for a couple of days. It was hard to turn the steering wheel sometimes. other times it felt fine. Thought is was the pump but when it steered easy the pump was quiet. When it steered hard the pump howled. After a couple of days the rack started pissing fluid out of the inner tie rod boot. Replace the rack and all was fine.
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337 C.I.D.
390 R.W.H.P.
362 R.W.T.Q
11.44@120 in the 1/4
7.35@ 95 in the 1/8
1.60-60'
N.A.
1988 L.X. Hatch 5.0L 5-Speed, 85,000 original miles
Could very well be the rack.. take a peek underneith and see if anything is out of the ordinary, such as, leaks, cracked or torn boot etc.. if thats the case replace as needed, and you should be good to go.. Good luck bro.
mustang racks suck!. call your local parts store i bet they have one in stock. not the funnest job in the world to do and a 59.00 alingnment afterwards is probably not what your looking foward too either