The 170's stopped appearing at Witham for several reasons - For a start, capacity was desperately needed on the Rural routes, especially on Norwich-Cambridge. The Lowestoft/Peterborough-Liverpool Street services were cut back to Ipswich, which freed up three units. These three 170/2 3-car's then transferred to take over the Norwich-Cambridge route giving a carriage extra of capacity on every service. The three 170/2 2-cars diagrammed for Norwich-Cambridge transferred to the Ipswich-Cambridge route to give a better passenger experience. The three Sprinters on this route then transferred to Norwich to give extra capacity on Norwich-Great Yarmouth/Lowestoft/Sheringham services, single 153's doubling up to pairs, 156's replacing single 153's etc etc.
Second of all was passenger capacity, as well as line capacity on the GEML south of Ipswich. The three-car 170's were very often rammed on shoulder-peak services, and with their slower acceleration, their stopping-pattern was generally limited to fast to Shenfield, Chelmsford, Witham, Marks Tey, Colchester and Ipswich to avoid getting in the way of EMU's and the Norwich's. The decision was taken to divert the London-Harwich service to Ipswich, with connections made to/from Harwich at Manningtree on Norwich services, and the Lowestoft/Peterborough services were withdrawn, and using unit's available off-peak, a through service in the same path was made available for an xx.38 Liverpool Street-Colchester Town, the reasoning being an 8/12-car EMU is far more effective at moving passengers than a 3-car 170. Also, the better acceleration of the EMU's and the more robust timetable allowed Kelvedon and Ingatestone to gain half-hourly services. The Hatfield Peverel stop was transferred from the Braintree's to the Ipswich's to speed up the Braintree's by a couple of minutes, with the additional Ingatestone stop inserted into the Clacton's, with the Romford stop previously in the Clacton's inserted into the new xx.38 Colchester Town service, as well as an extra Kelvedon stop.
170's occasionally make their way to Witham, one stood in for a 90 and Mark 3's a few months back at the last minute all the way through to Liverpool Street, and some work through to Ilford to visit the wheel lathe. A couple visit Colchester if you're interested, the last two Peterborough-Ipswich's run through in passenger service to Colchester for stabling.
Hope this helps!